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@ -7,6 +7,6 @@ This chapter describes the Linux kernel boot process. Here you will see a series
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* [Video mode initialization and transition to protected mode](linux-bootstrap-3.md) - describes video mode initialization in the kernel setup code and transition to protected mode.
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* [Transition to 64-bit mode](linux-bootstrap-4.md) - describes preparation for transition into 64-bit mode and details of transition.
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* [Kernel Decompression](linux-bootstrap-5.md) - describes preparation before kernel decompression and details of direct decompression.
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* [Kernel random address randomization](linux-bootstrap-6.md) - describes randomization of the Linux kernel load address.
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* [Kernel load address randomization](linux-bootstrap-6.md) - describes randomization of the Linux kernel load address.
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This chapter coincides with `Linux kernel v4.17`.
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@ -73,7 +73,7 @@ The starting address is formed by adding the base address to the value in the EI
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'0xfffffff0'
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```
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We get `0xfffffff0`, which is 16 bytes below 4GB. This point is called the [reset vector](https://en.wikipedia.org/wiki/Reset_vector). It's the memory location at which the CPU expects to find the first instruction to execute after reset. It contains a [jump](https://en.wikipedia.org/wiki/JMP_%28x86_instruction%29) (`jmp`) instruction that usually points to the [BIOS](https://en.wikipedia.org/wiki/BIOS) (Basic Input/Output System) entry point. For example, if we look in the [coreboot](https://www.coreboot.org/) source code (`src/cpu/x86/16bit/reset16.inc`), we see:
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We get `0xfffffff0`, which is 16 bytes below 4GB. This point is called the [reset vector](https://en.wikipedia.org/wiki/Reset_vector). It's the memory location at which the CPU expects to find the first instruction to execute after reset. It contains a [jump](https://en.wikipedia.org/wiki/JMP_%28x86_instruction%29) (`jmp`) instruction that usually points to the [BIOS](https://en.wikipedia.org/wiki/BIOS) (Basic Input/Output System) entry point. For example, if we look in the [coreboot](https://www.coreboot.org/) source code ([src/cpu/x86/16bit/reset16.inc](https://review.coreboot.org/plugins/gitiles/coreboot/+/refs/heads/4.11_branch/src/cpu/x86/16bit/reset16.inc)), we see:
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```assembly
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.section ".reset", "ax", %progbits
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@ -87,7 +87,7 @@ _start:
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Here we can see the `jmp` instruction [opcode](http://ref.x86asm.net/coder32.html#xE9), which is `0xe9`, and its destination address at `_start16bit - ( . + 2)`.
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We also see that the `reset` section is `16` bytes and is compiled to start from the address `0xfffffff0` (`src/cpu/x86/16bit/reset16.ld`):
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We also see that the `reset` section is `16` bytes and is compiled to start from the address `0xfffffff0` ([src/cpu/x86/16bit/reset16.ld](https://review.coreboot.org/plugins/gitiles/coreboot/+/refs/heads/4.11_branch/src/cpu/x86/16bit/reset16.ld)):
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```
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SECTIONS {
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@ -299,7 +299,7 @@ io_delay();
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At first, there is an inline assembly statement with a `cli` instruction which clears the interrupt flag (`IF`). After this, external interrupts are disabled. The next line disables NMI (non-maskable interrupt).
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An interrupt is a signal to the CPU which is emitted by hardware or software. After getting such a signal, the CPU suspends the current instruction sequence, saves its state and transfers control to the interrupt handler. After the interrupt handler has finished it's work, it transfers control back to the interrupted instruction. Non-maskable interrupts (NMI) are interrupts which are always processed, independently of permission. They cannot be ignored and are typically used to signal for non-recoverable hardware errors. We will not dive into the details of interrupts now but we will be discussing them in the coming posts.
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An interrupt is a signal to the CPU which is emitted by hardware or software. After getting such a signal, the CPU suspends the current instruction sequence, saves its state and transfers control to the interrupt handler. After the interrupt handler has finished its work, it transfers control back to the interrupted instruction. Non-maskable interrupts (NMI) are interrupts which are always processed, independently of permission. They cannot be ignored and are typically used to signal for non-recoverable hardware errors. We will not dive into the details of interrupts now but we will be discussing them in the coming posts.
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Let's get back to the code. We can see in the second line that we are writing the byte `0x80` (disabled bit) to `0x70` (the CMOS Address register). After that, a call to the `io_delay` function occurs. `io_delay` causes a small delay and looks like:
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@ -326,7 +326,7 @@ static int a20_test(int loops)
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saved = ctr = rdfs32(A20_TEST_ADDR);
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while (loops--) {
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while (loops--) {
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wrfs32(++ctr, A20_TEST_ADDR);
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io_delay(); /* Serialize and make delay constant */
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ok = rdgs32(A20_TEST_ADDR+0x10) ^ ctr;
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@ -204,7 +204,7 @@ Like before, we push `rsi` onto the stack to preserve the pointer to `boot_param
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* `output` - the start address of the decompressed kernel;
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* `output_len` - the size of the decompressed kernel;
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All arguments will be passed through registers as per the [System V Application Binary Interface](http://www.x86-64.org/documentation/abi.pdf). We've finished all the preparations and can now decompress the kernel.
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All arguments will be passed through registers as per the [System V Application Binary Interface](https://github.com/hjl-tools/x86-psABI/wiki/x86-64-psABI-1.0.pdf). We've finished all the preparations and can now decompress the kernel.
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Kernel decompression
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--------------------------------------------------------------------------------
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@ -46,7 +46,7 @@ This function takes five parameters:
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* `input`;
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* `input_size`;
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* `output`;
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* `output_isze`;
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* `output_size`;
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* `virt_addr`.
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Let's try to understand what these parameters are. The first parameter, `input` is just the `input_data` parameter of the `extract_kernel` function from the [arch/x86/boot/compressed/misc.c](https://github.com/torvalds/linux/blob/v4.16/arch/x86/boot/compressed/misc.c) source code file, cast to `unsigned long`:
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@ -146,7 +146,7 @@ Now, we call another function:
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initialize_identity_maps();
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```
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The `initialize_identity_maps` function is defined in the [arch/x86/boot/compressed/kaslr_64.c](https://github.com/torvalds/linux/blob/master/arch/x86/boot/compressed/kaslr_64.c) source code file. This function starts by initialising an instance of the `x86_mapping_info` structure called `mapping_info`:
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The `initialize_identity_maps` function is defined in the [arch/x86/boot/compressed/kaslr_64.c](https://github.com/torvalds/linux/blob/master/arch/x86/boot/compressed/kaslr_64.c) source code file. This function starts by initializing an instance of the `x86_mapping_info` structure called `mapping_info`:
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|
||||
```C
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mapping_info.alloc_pgt_page = alloc_pgt_page;
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@ -254,7 +254,7 @@ add_identity_map(mem_avoid[MEM_AVOID_ZO_RANGE].start,
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mem_avoid[MEM_AVOID_ZO_RANGE].size);
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```
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THe `mem_avoid_init` function first tries to avoid memory regions currently used to decompress the kernel. We fill an entry from the `mem_avoid` array with the start address and the size of the relevant region and call the `add_identity_map` function, which builds the identity mapped pages for this region. The `add_identity_map` function is defined in the [arch/x86/boot/compressed/kaslr_64.c](https://github.com/torvalds/linux/blob/v4.16/arch/x86/boot/compressed/kaslr_64.c) source code file and looks like this:
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The `mem_avoid_init` function first tries to avoid memory regions currently used to decompress the kernel. We fill an entry from the `mem_avoid` array with the start address and the size of the relevant region and call the `add_identity_map` function, which builds the identity mapped pages for this region. The `add_identity_map` function is defined in the [arch/x86/boot/compressed/kaslr_64.c](https://github.com/torvalds/linux/blob/v4.16/arch/x86/boot/compressed/kaslr_64.c) source code file and looks like this:
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|
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```C
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void add_identity_map(unsigned long start, unsigned long size)
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@ -30,7 +30,7 @@ Each of these control group subsystems depends on related configuration option.
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You may see enabled control groups on your computer via [proc](https://en.wikipedia.org/wiki/Procfs) filesystem:
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```
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$ cat /proc/cgroups
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$ cat /proc/cgroups
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#subsys_name hierarchy num_cgroups enabled
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cpuset 8 1 1
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cpu 7 66 1
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@ -90,7 +90,7 @@ So, if we will run this script we will see following result:
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||||
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||||
```
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$ sudo chmod +x cgroup_test_script.sh
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~$ ./cgroup_test_script.sh
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~$ ./cgroup_test_script.sh
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print line
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print line
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print line
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@ -147,7 +147,7 @@ crw-rw-rw- 1 root tty 5, 0 Dec 3 22:48 /dev/tty
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see the first `c` letter in a permissions list. The second part is `5:0` is major and minor numbers of the device. You can see these numbers in the output of `ls` too. And the last `w` letter forbids tasks to write to the specified device. So let's start the `cgroup_test_script.sh` script:
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```
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~$ ./cgroup_test_script.sh
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~$ ./cgroup_test_script.sh
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print line
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print line
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print line
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@ -164,7 +164,7 @@ and add pid of this process to the `devices/tasks` file of our group:
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The result of this action will be as expected:
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```
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~$ ./cgroup_test_script.sh
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~$ ./cgroup_test_script.sh
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print line
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print line
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print line
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@ -174,7 +174,7 @@ print line
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./cgroup_test_script.sh: line 5: /dev/tty: Operation not permitted
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```
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Similar situation will be when you will run you [docker](https://en.wikipedia.org/wiki/Docker_\(software\)) containers for example:
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||||
Similar situation will be when you will run you [docker](https://en.wikipedia.org/wiki/Docker_(software)) containers for example:
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||||
|
||||
```
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~$ docker ps
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@ -213,7 +213,7 @@ Control group /:
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│ └─6404 /bin/bash
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```
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Now we know a little about `control groups` mechanism, how to use it manually and what's purpose of this mechanism. It's time to look inside of the Linux kernel source code and start to dive into implementation of this mechanism.
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Now we know a little about `control groups` mechanism, how to use it manually and what's the purpose of this mechanism. It's time to look inside of the Linux kernel source code and start to dive into implementation of this mechanism.
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||||
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Early initialization of control groups
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--------------------------------------------------------------------------------
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@ -294,7 +294,7 @@ Here we may see call of the `init_cgroup_root` function which will execute initi
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struct cgroup_root cgrp_dfl_root;
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```
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Its `cgrp` field represented by the `cgroup` structure which represents a `cgroup` as you already may guess and defined in the [include/linux/cgroup-defs.h](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/include/linux/cgroup-defs.h) header file. We already know that a process which is represented by the `task_struct` in the Linux kernel. The `task_struct` does not contain direct link to a `cgroup` where this task is attached. But it may be reached via `css_set` field of the `task_struct`. This `css_set` structure holds pointer to the array of subsystem states:
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||||
Its `cgrp` field represented by the `cgroup` structure which represents a `cgroup` as you already may guess and defined in the [include/linux/cgroup-defs.h](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/include/linux/cgroup-defs.h) header file. We already know that a process is represented by the `task_struct` in the Linux kernel. The `task_struct` does not contain direct link to a `cgroup` where this task is attached. But it may be reached via `css_set` field of the `task_struct`. This `css_set` structure holds pointer to the array of subsystem states:
|
||||
|
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```C
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struct css_set {
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@ -324,14 +324,14 @@ struct cgroup_subsys_state {
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||||
So, the overall picture of `cgroups` related data structure is following:
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```
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```
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+-------------+ +---------------------+ +------------->+---------------------+ +----------------+
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||||
| task_struct | | css_set | | | cgroup_subsys_state | | cgroup |
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+-------------+ | | | +---------------------+ +----------------+
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||||
| | | | | | | | flags |
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||||
| | | | | +---------------------+ | cgroup.procs |
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| | | | | | cgroup |--------->| id |
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||||
| | | | | +---------------------+ | .... |
|
||||
| | | | | +---------------------+ | .... |
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||||
|-------------+ |---------------------+----+ +----------------+
|
||||
| cgroups | ------> | cgroup_subsys_state | array of cgroup_subsys_state
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|-------------+ +---------------------+------------------>+---------------------+ +----------------+
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|
||||
@ -19,13 +19,13 @@ set_cpu_present(cpu, true);
|
||||
set_cpu_possible(cpu, true);
|
||||
```
|
||||
|
||||
Before we will consider implementation of these functions, let's consider all of these masks.
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Before we consider implementation of these functions, let's consider all of these masks.
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|
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The `cpu_possible` is a set of cpu ID's which can be plugged in anytime during the life of that system boot or in other words mask of possible CPUs contains maximum number of CPUs which are possible in the system. It will be equal to value of the `NR_CPUS` which is which is set statically via the `CONFIG_NR_CPUS` kernel configuration option.
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The `cpu_possible` is a set of cpu ID's which can be plugged in anytime during the life of that system boot or in other words mask of possible CPUs contains maximum number of CPUs which are possible in the system. It will be equal to value of the `NR_CPUS` which is set statically via the `CONFIG_NR_CPUS` kernel configuration option.
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|
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The `cpu_present` mask represents which CPUs are currently plugged in.
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The `cpu_online` represents a subset of the `cpu_present` and indicates CPUs which are available for scheduling or in other words a bit from this mask tells to kernel is a processor may be utilized by the Linux kernel.
|
||||
The `cpu_online` represents a subset of the `cpu_present` and indicates CPUs which are available for scheduling or in other words a bit from this mask tells the kernel if a processor may be utilized by the Linux kernel.
|
||||
|
||||
The last mask is `cpu_active`. Bits of this mask tells to Linux kernel is a task may be moved to a certain processor.
|
||||
|
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@ -94,9 +94,9 @@ And returns `1` every time. We need it here for only one purpose: at compile tim
|
||||
cpumask API
|
||||
--------------------------------------------------------------------------------
|
||||
|
||||
As we can define cpumask with one of the method, Linux kernel provides API for manipulating a cpumask. Let's consider one of the function which presented above. For example `set_cpu_online`. This function takes two parameters:
|
||||
As we can define cpumask with one of the methods, Linux kernel provides API for manipulating a cpumask. Let's consider one of the function which presented above. For example `set_cpu_online`. This function takes two parameters:
|
||||
|
||||
* Number of CPU;
|
||||
* Index of CPU;
|
||||
* CPU status;
|
||||
|
||||
Implementation of this function looks as:
|
||||
@ -113,7 +113,7 @@ void set_cpu_online(unsigned int cpu, bool online)
|
||||
}
|
||||
```
|
||||
|
||||
First of all it checks the second `state` parameter and calls `cpumask_set_cpu` or `cpumask_clear_cpu` depends on it. Here we can see casting to the `struct cpumask *` of the second parameter in the `cpumask_set_cpu`. In our case it is `cpu_online_bits` which is a bitmap and defined as:
|
||||
First of all it checks the second `state` parameter and calls `cpumask_set_cpu` or `cpumask_clear_cpu` depending on it. Here we can see casting to the `struct cpumask *` of the second parameter in the `cpumask_set_cpu`. In our case it is `cpu_online_bits` which is a bitmap and defined as:
|
||||
|
||||
```C
|
||||
static DECLARE_BITMAP(cpu_online_bits, CONFIG_NR_CPUS) __read_mostly;
|
||||
@ -128,7 +128,7 @@ static inline void cpumask_set_cpu(unsigned int cpu, struct cpumask *dstp)
|
||||
}
|
||||
```
|
||||
|
||||
The `set_bit` function takes two parameters too, and sets a given bit (first parameter) in the memory (second parameter or `cpu_online_bits` bitmap). We can see here that before `set_bit` will be called, its two parameters will be passed to the
|
||||
The `set_bit` function takes two parameters too, and sets a given bit (first parameter) in the memory (second parameter or `cpu_online_bits` bitmap). We can see here that before `set_bit` is called, its two parameters will be passed to the
|
||||
|
||||
* cpumask_check;
|
||||
* cpumask_bits.
|
||||
|
||||
@ -6,12 +6,12 @@ Introduction
|
||||
|
||||
The Linux kernel is huge piece of [C](https://en.wikipedia.org/wiki/C_%28programming_language%29) code which consists from many different subsystems. Each subsystem has its own purpose which is independent of other subsystems. But often one subsystem wants to know something from other subsystem(s). There is special mechanism in the Linux kernel which allows to solve this problem partly. The name of this mechanism is - `notification chains` and its main purpose to provide a way for different subsystems to subscribe on asynchronous events from other subsystems. Note that this mechanism is only for communication inside kernel, but there are other mechanisms for communication between kernel and userspace.
|
||||
|
||||
Before we will consider `notification chains` [API](https://en.wikipedia.org/wiki/Application_programming_interface) and implementation of this API, let's look at `Notification chains` mechanism from theoretical side as we did it in other parts of this book. Everything which is related to `notification chains` mechanism is located in the [include/linux/notifier.h](https://github.com/torvalds/linux/blob/master/include/linux/notifier.h) header file and [kernel/notifier.c](https://github.com/torvalds/linux/blob/master/kernel/notifier.c) source code file. So let's open them and start to dive.
|
||||
Before we consider `notification chains` [API](https://en.wikipedia.org/wiki/Application_programming_interface) and implementation of this API, let's look at `Notification chains` mechanism from theoretical side as we did it in other parts of this book. Everything which is related to `notification chains` mechanism is located in the [include/linux/notifier.h](https://github.com/torvalds/linux/blob/master/include/linux/notifier.h) header file and [kernel/notifier.c](https://github.com/torvalds/linux/blob/master/kernel/notifier.c) source code file. So let's open them and start to dive.
|
||||
|
||||
Notification Chains related data structures
|
||||
--------------------------------------------------------------------------------
|
||||
|
||||
Let's start to consider `notification chains` mechanism from related data structures. As I wrote above, main data structures should be located in the [include/linux/notifier.h](https://github.com/torvalds/linux/blob/master/include/linux/notifier.h) header file, so the Linux kernel provides generic API which does not depend on certain architecture. In general, the `notification chains` mechanism represents a list (that's why it named `chains`) of [callback](https://en.wikipedia.org/wiki/Callback_%28computer_programming%29) functions which are will be executed when an event will be occurred.
|
||||
Let's start to consider `notification chains` mechanism from related data structures. As I wrote above, main data structures should be located in the [include/linux/notifier.h](https://github.com/torvalds/linux/blob/master/include/linux/notifier.h) header file, so the Linux kernel provides generic API which does not depend on certain architecture. In general, the `notification chains` mechanism represents a list (that's why it's named `chains`) of [callback](https://en.wikipedia.org/wiki/Callback_%28computer_programming%29) functions which are will be executed when an event will be occurred.
|
||||
|
||||
All of these callback functions are represented as `notifier_fn_t` type in the Linux kernel:
|
||||
|
||||
@ -101,7 +101,7 @@ Now as we know a little about `notification chains` mechanism let's consider imp
|
||||
Notification Chains
|
||||
--------------------------------------------------------------------------------
|
||||
|
||||
Usually there are two sides in a publish/subscriber mechanisms. One side who wants to get notifications and other side(s) who generates these notifications. We will consider notification chains mechanism from both sides. We will consider `blocking notification chains` in this part, because of other types of notification chains are similar to it and differs mostly in protection mechanisms.
|
||||
Usually there are two sides in a publish/subscriber mechanisms. One side who wants to get notifications and other side(s) who generates these notifications. We will consider notification chains mechanism from both sides. We will consider `blocking notification chains` in this part, because of other types of notification chains are similar to it and differ mostly in protection mechanisms.
|
||||
|
||||
Before a notification producer is able to produce notification, first of all it should initialize head of a notification chain. For example let's consider notification chains related to kernel [loadable modules](https://en.wikipedia.org/wiki/Loadable_kernel_module). If we will look in the [kernel/module.c](https://github.com/torvalds/linux/blob/master/kernel/module.c) source code file, we will see following definition:
|
||||
|
||||
@ -120,7 +120,7 @@ which defines head for loadable modules blocking notifier chain. The `BLOCKING_N
|
||||
|
||||
So we may see that it takes name of a name of a head of a blocking notifier chain and initializes read/write [semaphore](https://0xax.gitbook.io/linux-insides/summary/syncprim/linux-sync-3) and set head to `NULL`. Besides the `BLOCKING_INIT_NOTIFIER_HEAD` macro, the Linux kernel additionally provides `ATOMIC_INIT_NOTIFIER_HEAD`, `RAW_INIT_NOTIFIER_HEAD` macros and `srcu_init_notifier` function for initialization atomic and other types of notification chains.
|
||||
|
||||
After initialization of a head of a notification chain, a subsystem which wants to receive notification from the given notification chain it should register with certain function which is depends on type of notification. If you will look in the [include/linux/notifier.h](https://github.com/torvalds/linux/blob/master/include/linux/notifier.h) header file, you will see following four function for this:
|
||||
After initialization of a head of a notification chain, a subsystem which wants to receive notification from the given notification chain should register with certain function which depends on the type of notification. If you will look in the [include/linux/notifier.h](https://github.com/torvalds/linux/blob/master/include/linux/notifier.h) header file, you will see following four function for this:
|
||||
|
||||
```C
|
||||
extern int atomic_notifier_chain_register(struct atomic_notifier_head *nh,
|
||||
@ -247,7 +247,7 @@ int __blocking_notifier_call_chain(struct blocking_notifier_head *nh,
|
||||
}
|
||||
```
|
||||
|
||||
Where `nr_to_call` and `nr_calls` are number of notifier functions to be called and number of sent notifications. As you may guess the main goal of the `__blocking_notifer_call_chain` function and other functions for other notification types is to call callback function when an event occurred. Implementation of the `__blocking_notifier_call_chain` is pretty simple, it just calls the `notifier_call_chain` function from the same source code file protected with read/write semaphore:
|
||||
Where `nr_to_call` and `nr_calls` are number of notifier functions to be called and number of sent notifications. As you may guess the main goal of the `__blocking_notifer_call_chain` function and other functions for other notification types is to call callback function when an event occurs. Implementation of the `__blocking_notifier_call_chain` is pretty simple, it just calls the `notifier_call_chain` function from the same source code file protected with read/write semaphore:
|
||||
|
||||
```C
|
||||
int __blocking_notifier_call_chain(struct blocking_notifier_head *nh,
|
||||
@ -266,7 +266,7 @@ int __blocking_notifier_call_chain(struct blocking_notifier_head *nh,
|
||||
}
|
||||
```
|
||||
|
||||
and returns its result. In this case all job is done by the `notifier_call_chain` function. Main purpose of this function informs registered notifiers about an asynchronous event:
|
||||
and returns its result. In this case all job is done by the `notifier_call_chain` function. Main purpose of this function is to inform registered notifiers about an asynchronous event:
|
||||
|
||||
```C
|
||||
static int notifier_call_chain(struct notifier_block **nl,
|
||||
@ -298,7 +298,7 @@ definition of the `module_notify_list` in the [kernel/module.c](https://github.c
|
||||
* MODULE_STATE_COMING
|
||||
* MODULE_STATE_GOING
|
||||
|
||||
in which maybe interested some subsystems of the Linux kernel. For example tracing of kernel modules states. Instead of direct call of the `atomic_notifier_chain_register`, `blocking_notifier_chain_register` and etc., most notification chains come with a set of wrappers used to register to them. Registatrion on these modules events is going with the help of such wrapper:
|
||||
in which maybe interested some subsystems of the Linux kernel. For example tracing of kernel modules states. Instead of direct call of the `atomic_notifier_chain_register`, `blocking_notifier_chain_register` and etc., most notification chains come with a set of wrappers used to register to them. Registration on these modules events is going with the help of such wrapper:
|
||||
|
||||
```C
|
||||
int register_module_notifier(struct notifier_block *nb)
|
||||
@ -348,7 +348,7 @@ SYSCALL_DEFINE2(delete_module, const char __user *, name_user,
|
||||
}
|
||||
```
|
||||
|
||||
Thus when one of these system call will be called from userspace, the Linux kernel will send certain notification depends on a system call and the `tracepoint_module_notify` callback function will be called.
|
||||
Thus when one of these system call will be called from userspace, the Linux kernel will send certain notification depending on a system call and the `tracepoint_module_notify` callback function will be called.
|
||||
|
||||
That's all.
|
||||
|
||||
|
||||
@ -9,11 +9,11 @@ Besides different [linked](https://en.wikipedia.org/wiki/Linked_data_structure)
|
||||
* [lib/bitmap.c](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/lib/bitmap.c)
|
||||
* [include/linux/bitmap.h](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/include/linux/bitmap.h)
|
||||
|
||||
Besides these two files, there is also architecture-specific header file which provides optimized bit operations for certain architecture. We consider [x86_64](https://en.wikipedia.org/wiki/X86-64) architecture, so in our case it will be:
|
||||
Besides these two files, there is also architecture-specific header file which provides optimized bit operations for certain architecture. We consider [x86_64](https://en.wikipedia.org/wiki/X86-64) architecture, so in our case it will be:
|
||||
|
||||
* [arch/x86/include/asm/bitops.h](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/arch/x86/include/asm/bitops.h)
|
||||
|
||||
header file. As I just wrote above, the `bitmap` is heavily used in the Linux kernel. For example a `bit array` is used to store set of online/offline processors for systems which support [hot-plug](https://www.kernel.org/doc/Documentation/cpu-hotplug.txt) cpu (more about this you can read in the [cpumasks](https://0xax.gitbook.io/linux-insides/summary/concepts/linux-cpu-2) part), a `bit array` stores set of allocated [irqs](https://en.wikipedia.org/wiki/Interrupt_request_%28PC_architecture%29) during initialization of the Linux kernel and etc.
|
||||
header file. As I just wrote above, the `bitmap` is heavily used in the Linux kernel. For example a `bit array` is used to store set of online/offline processors for systems which support [hot-plug](https://www.kernel.org/doc/Documentation/cpu-hotplug.txt) CPU (more about this you can read in the [cpumasks](https://0xax.gitbook.io/linux-insides/summary/concepts/linux-cpu-2) part), a `bit array` stores set of allocated [IRQs](https://en.wikipedia.org/wiki/Interrupt_request_%28PC_architecture%29) during initialization of the Linux kernel and etc.
|
||||
|
||||
So, the main goal of this part is to see how `bit arrays` are implemented in the Linux kernel. Let's start.
|
||||
|
||||
@ -75,7 +75,7 @@ I think that there is no need to explain what these function do. This is already
|
||||
|
||||
In simple words atomic operations guarantees that two or more operations will not be performed on the same data concurrently. The `x86` architecture provides a set of atomic instructions, for example [xchg](http://x86.renejeschke.de/html/file_module_x86_id_328.html) instruction, [cmpxchg](http://x86.renejeschke.de/html/file_module_x86_id_41.html) instruction and etc. Besides atomic instructions, some of non-atomic instructions can be made atomic with the help of the [lock](http://x86.renejeschke.de/html/file_module_x86_id_159.html) instruction. It is enough to know about atomic operations for now, so we can begin to consider implementation of `set_bit` and `clear_bit` functions.
|
||||
|
||||
First of all, let's start to consider `non-atomic` variants of this function. Names of non-atomic `set_bit` and `clear_bit` starts from double underscore. As we already know, all of these functions are defined in the [arch/x86/include/asm/bitops.h](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/arch/x86/include/asm/bitops.h) header file and the first function is `__set_bit`:
|
||||
First of all, let's start to consider `non-atomic` variants of this function. Names of non-atomic `set_bit` and `clear_bit` starts with double underscore. As we already know, all of these functions are defined in the [arch/x86/include/asm/bitops.h](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/arch/x86/include/asm/bitops.h) header file and the first function is `__set_bit`:
|
||||
|
||||
```C
|
||||
static inline void __set_bit(long nr, volatile unsigned long *addr)
|
||||
@ -134,13 +134,13 @@ which means that this function will be always inlined to reduce size of the Linu
|
||||
#define IS_IMMEDIATE(nr) (__builtin_constant_p(nr))
|
||||
```
|
||||
|
||||
The `__builtin_constant_p` builtin function returns `1` if the given parameter is known to be constant at compile-time and returns `0` in other case. We no need to use slow `bts` instruction to set bit if the given number of bit is known in compile time constant. We can just apply [bitwise or](https://en.wikipedia.org/wiki/Bitwise_operation#OR) for byte from the give address which contains given bit and masked number of bits where high bit is `1` and other is zero. In other case if the given number of bit is not known constant at compile-time, we do the same as we did in the `__set_bit` function. The `CONST_MASK_ADDR` macro:
|
||||
The `__builtin_constant_p` builtin function returns `1` if the given parameter is known to be constant at compile-time and returns `0` in other case. We do not need to use slow `bts` instruction to set bit if the given number of bit is known in compile time constant. We can just apply [bitwise or](https://en.wikipedia.org/wiki/Bitwise_operation#OR) for byte from the give address which contains given bit and masked number of bits where high bit is `1` and other is zero. In other case if the given number of bit is not known constant at compile-time, we do the same as we did in the `__set_bit` function. The `CONST_MASK_ADDR` macro:
|
||||
|
||||
```C
|
||||
#define CONST_MASK_ADDR(nr, addr) BITOP_ADDR((void *)(addr) + ((nr)>>3))
|
||||
```
|
||||
|
||||
expands to the give address with offset to the byte which contains a given bit. For example we have address `0x1000` and the number of bit is `0x9`. So, as `0x9` is `one byte + one bit` our address with be `addr + 1`:
|
||||
expands to the given address with offset to the byte which contains a given bit. For example we have address `0x1000` and the number of bit is `0x9`. So, as `0x9` is `one byte + one bit` our address with be `addr + 1`:
|
||||
|
||||
```python
|
||||
>>> hex(0x1000 + (0x9 >> 3))
|
||||
@ -204,7 +204,7 @@ and as we can see it is very similar on `set_bit` and just contains two differen
|
||||
|
||||
That's all. Now we can set and clear bit in any bit array and and we can go to other operations on bitmasks.
|
||||
|
||||
Most widely used operations on a bit arrays are set and clear bit in a bit array in the Linux kernel. But besides this operations it is useful to do additional operations on a bit array. Yet another widely used operation in the Linux kernel - is to know is a given bit set or not in a bit array. We can achieve this with the help of the `test_bit` macro. This macro is defined in the [arch/x86/include/asm/bitops.h](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/arch/x86/include/asm/bitops.h) header file and expands to the call of the `constant_test_bit` or `variable_test_bit` depends on bit number:
|
||||
Most widely used operations on a bit arrays are set and clear bit in a bit array in the Linux kernel. But besides this operations it is useful to do additional operations on a bit array. Yet another widely used operation in the Linux kernel - is to know if a given bit is set or not in a bit array. We can achieve this with the help of the `test_bit` macro. This macro is defined in the [arch/x86/include/asm/bitops.h](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/arch/x86/include/asm/bitops.h) header file and expands to the call of the `constant_test_bit` or `variable_test_bit` depending on bit number:
|
||||
|
||||
```C
|
||||
#define test_bit(nr, addr) \
|
||||
@ -257,7 +257,7 @@ static inline void __change_bit(long nr, volatile unsigned long *addr)
|
||||
}
|
||||
```
|
||||
|
||||
Pretty easy, is not it? The implementation of the `__change_bit` is the same as `__set_bit`, but instead of `bts` instruction, we are using [btc](http://x86.renejeschke.de/html/file_module_x86_id_23.html). This instruction selects a given bit from a given bit array, stores its value in the `CF` and changes its value by the applying of complement operation. So, a bit with value `1` will be `0` and vice versa:
|
||||
Pretty easy, is it not? The implementation of the `__change_bit` is the same as `__set_bit`, but instead of `bts` instruction, we are using [btc](http://x86.renejeschke.de/html/file_module_x86_id_23.html). This instruction selects a given bit from a given bit array, stores its value in the `CF` and changes its value by the applying of complement operation. So, a bit with value `1` will be `0` and vice versa:
|
||||
|
||||
```python
|
||||
>>> int(not 1)
|
||||
@ -290,7 +290,7 @@ For this moment we know the most important architecture-specific operations with
|
||||
Common bit operations
|
||||
================================================================================
|
||||
|
||||
Besides the architecture-specific API from the [arch/x86/include/asm/bitops.h](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/arch/x86/include/asm/bitops.h) header file, the Linux kernel provides common API for manipulation of bit arrays. As we know from the beginning of this part, we can find it in the [include/linux/bitmap.h](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/include/linux/bitmap.h) header file and additionally in the * [lib/bitmap.c](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/lib/bitmap.c) source code file. But before these source code files let's look into the [include/linux/bitops.h](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/include/linux/bitops.h) header file which provides a set of useful macro. Let's look on some of they.
|
||||
Besides the architecture-specific API from the [arch/x86/include/asm/bitops.h](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/arch/x86/include/asm/bitops.h) header file, the Linux kernel provides common API for manipulation of bit arrays. As we know from the beginning of this part, we can find it in the [include/linux/bitmap.h](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/include/linux/bitmap.h) header file and additionally in the [lib/bitmap.c](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/lib/bitmap.c) source code file. But before these source code files let's look into the [include/linux/bitops.h](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/include/linux/bitops.h) header file which provides a set of useful macro. Let's look on some of them.
|
||||
|
||||
First of all let's look at following four macros:
|
||||
|
||||
@ -317,7 +317,7 @@ The next [header](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2
|
||||
* `bitmap_zero`;
|
||||
* `bitmap_fill`.
|
||||
|
||||
To clear a bit array and fill it with `1`. Let's look on the implementation of the `bitmap_zero` function:
|
||||
To clear a bit array or fill it with `1`. Let's look at the implementation of the `bitmap_zero` function:
|
||||
|
||||
```C
|
||||
static inline void bitmap_zero(unsigned long *dst, unsigned int nbits)
|
||||
@ -340,7 +340,7 @@ First of all we can see the check for `nbits`. The `small_const_nbits` is macro
|
||||
|
||||
As we may see it checks that `nbits` is known constant in compile time and `nbits` value does not overflow `BITS_PER_LONG` or `64`. If bits number does not overflow amount of bits in a `long` value we can just set to zero. In other case we need to calculate how many `long` values do we need to fill our bit array and fill it with [memset](http://man7.org/linux/man-pages/man3/memset.3.html).
|
||||
|
||||
The implementation of the `bitmap_fill` function is similar on implementation of the `biramp_zero` function, except we fill a given bit array with `0xff` values or `0b11111111`:
|
||||
The implementation of the `bitmap_fill` function is similar on implementation of the `bitmap_zero` function, except we fill a given bit array with `0xff` values or `0b11111111`:
|
||||
|
||||
```C
|
||||
static inline void bitmap_fill(unsigned long *dst, unsigned int nbits)
|
||||
@ -354,7 +354,7 @@ static inline void bitmap_fill(unsigned long *dst, unsigned int nbits)
|
||||
}
|
||||
```
|
||||
|
||||
Besides the `bitmap_fill` and `bitmap_zero` functions, the [include/linux/bitmap.h](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/include/linux/bitmap.h) header file provides `bitmap_copy` which is similar on the `bitmap_zero`, but just uses [memcpy](http://man7.org/linux/man-pages/man3/memcpy.3.html) instead of [memset](http://man7.org/linux/man-pages/man3/memset.3.html). Also it provides bitwise operations for bit array like `bitmap_and`, `bitmap_or`, `bitamp_xor` and etc. We will not consider implementation of these functions because it is easy to understand implementations of these functions if you understood all from this part. Anyway if you are interested how did these function implemented, you may open [include/linux/bitmap.h](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/include/linux/bitmap.h) header file and start to research.
|
||||
Besides the `bitmap_fill` and `bitmap_zero` functions, the [include/linux/bitmap.h](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/include/linux/bitmap.h) header file provides `bitmap_copy` which is similar on the `bitmap_zero`, but just uses [memcpy](http://man7.org/linux/man-pages/man3/memcpy.3.html) instead of [memset](http://man7.org/linux/man-pages/man3/memset.3.html). Also it provides bitwise operations for bit array like `bitmap_and`, `bitmap_or`, `bitamp_xor` and etc. We will not consider implementation of these functions because it is easy to understand implementations of these functions if you understood all from this part. Anyway if you are interested in how these function are implemented, you may open [include/linux/bitmap.h](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/include/linux/bitmap.h) header file and start to research.
|
||||
|
||||
That's all.
|
||||
|
||||
@ -363,7 +363,7 @@ Links
|
||||
|
||||
* [bitmap](https://en.wikipedia.org/wiki/Bit_array)
|
||||
* [linked data structures](https://en.wikipedia.org/wiki/Linked_data_structure)
|
||||
* [tree data structures](https://en.wikipedia.org/wiki/Tree_%28data_structure%29)
|
||||
* [tree data structures](https://en.wikipedia.org/wiki/Tree_%28data_structure%29)
|
||||
* [hot-plug](https://www.kernel.org/doc/Documentation/cpu-hotplug.txt)
|
||||
* [cpumasks](https://0xax.gitbook.io/linux-insides/summary/concepts/linux-cpu-2)
|
||||
* [IRQs](https://en.wikipedia.org/wiki/Interrupt_request_%28PC_architecture%29)
|
||||
@ -377,7 +377,7 @@ Links
|
||||
* [bt instruction](http://x86.renejeschke.de/html/file_module_x86_id_22.html)
|
||||
* [sbb instruction](http://x86.renejeschke.de/html/file_module_x86_id_286.html)
|
||||
* [btc instruction](http://x86.renejeschke.de/html/file_module_x86_id_23.html)
|
||||
* [man memcpy](http://man7.org/linux/man-pages/man3/memcpy.3.html)
|
||||
* [man memcpy](http://man7.org/linux/man-pages/man3/memcpy.3.html)
|
||||
* [man memset](http://man7.org/linux/man-pages/man3/memset.3.html)
|
||||
* [CF](https://en.wikipedia.org/wiki/FLAGS_register)
|
||||
* [inline assembler](https://en.wikipedia.org/wiki/Inline_assembler)
|
||||
|
||||
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|
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|
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|
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@ -132,12 +132,12 @@ structure from the [include/uapi/linux/resource.h](https://github.com/torvalds/l
|
||||
|
||||
```C
|
||||
cat /proc/self/limits
|
||||
Limit Soft Limit Hard Limit Units
|
||||
Limit Soft Limit Hard Limit Units
|
||||
...
|
||||
...
|
||||
...
|
||||
Max processes 63815 63815 processes
|
||||
Max pending signals 63815 63815 signals
|
||||
Max processes 63815 63815 processes
|
||||
Max pending signals 63815 63815 signals
|
||||
...
|
||||
...
|
||||
...
|
||||
@ -149,7 +149,7 @@ Initialization of the caches
|
||||
The next function after the `fork_init` is the `proc_caches_init` from the [kernel/fork.c](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/kernel/fork.c). This function allocates caches for the memory descriptors (or `mm_struct` structure). At the beginning of the `proc_caches_init` we can see allocation of the different [SLAB](http://en.wikipedia.org/wiki/Slab_allocation) caches with the call of the `kmem_cache_create`:
|
||||
|
||||
* `sighand_cachep` - manage information about installed signal handlers;
|
||||
* `signal_cachep` - manage information about process signal descriptor;
|
||||
* `signal_cachep` - manage information about process signal descriptor;
|
||||
* `files_cachep` - manage information about opened files;
|
||||
* `fs_cachep` - manage filesystem information.
|
||||
|
||||
@ -230,7 +230,7 @@ and a couple of directories depends on the different configuration options:
|
||||
|
||||
In the end of the `proc_root_init` we call the `proc_sys_init` function which creates `/proc/sys` directory and initializes the [Sysctl](http://en.wikipedia.org/wiki/Sysctl).
|
||||
|
||||
It is the end of `start_kernel` function. I did not describe all functions which are called in the `start_kernel`. I skipped them, because they are not important for the generic kernel initialization stuff and depend on only different kernel configurations. They are `taskstats_init_early` which exports per-task statistic to the user-space, `delayacct_init` - initializes per-task delay accounting, `key_init` and `security_init` initialize different security stuff, `check_bugs` - fix some architecture-dependent bugs, `ftrace_init` function executes initialization of the [ftrace](https://www.kernel.org/doc/Documentation/trace/ftrace.txt), `cgroup_init` makes initialization of the rest of the [cgroup](http://en.wikipedia.org/wiki/Cgroups) subsystem,etc. Many of these parts and subsystems will be described in the other chapters.
|
||||
It is the end of `start_kernel` function. I did not describe all functions which are called in the `start_kernel`. I skipped them, because they are not important for the generic kernel initialization stuff and depend on only different kernel configurations. They are `taskstats_init_early` which exports per-task statistic to the user-space, `delayacct_init` - initializes per-task delay accounting, `key_init` and `security_init` initialize different security stuff, `check_bugs` - fix some architecture-dependent bugs, `ftrace_init` function executes initialization of the [ftrace](https://www.kernel.org/doc/Documentation/trace/ftrace.txt), `cgroup_init` makes initialization of the rest of the [cgroup](http://en.wikipedia.org/wiki/Cgroups) subsystem, etc. Many of these parts and subsystems will be described in the other chapters.
|
||||
|
||||
That's all. Finally we have passed through the long-long `start_kernel` function. But it is not the end of the Linux kernel initialization process. We haven't run the first process yet. In the end of the `start_kernel` we can see the last call of the - `rest_init` function. Let's go ahead.
|
||||
|
||||
@ -251,7 +251,7 @@ kernel_thread(kernel_init, NULL, CLONE_FS);
|
||||
pid = kernel_thread(kthreadd, NULL, CLONE_FS | CLONE_FILES);
|
||||
```
|
||||
|
||||
Here the `kernel_thread` function (defined in the [kernel/fork.c](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/kernel/fork.c)) creates new kernel thread.As we can see the `kernel_thread` function takes three arguments:
|
||||
Here the `kernel_thread` function (defined in the [kernel/fork.c](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/kernel/fork.c)) creates new kernel thread. As we can see the `kernel_thread` function takes three arguments:
|
||||
|
||||
* Function which will be executed in a new thread;
|
||||
* Parameter for the `kernel_init` function;
|
||||
@ -271,7 +271,7 @@ Let's postpone `kernel_init` and `kthreadd` for now and go ahead in the `rest_in
|
||||
kthreadd_task = find_task_by_pid_ns(pid, &init_pid_ns);
|
||||
rcu_read_unlock();
|
||||
```
|
||||
|
||||
|
||||
The first `rcu_read_lock` function marks the beginning of an [RCU](http://en.wikipedia.org/wiki/Read-copy-update) read-side critical section and the `rcu_read_unlock` marks the end of an RCU read-side critical section. We call these functions because we need to protect the `find_task_by_pid_ns`. The `find_task_by_pid_ns` returns pointer to the `task_struct` by the given pid. So, here we are getting the pointer to the `task_struct` for `PID = 2` (we got it after `kthreadd` creation with the `kernel_thread`). In the next step we call `complete` function
|
||||
|
||||
```C
|
||||
@ -314,7 +314,7 @@ void init_idle_bootup_task(struct task_struct *idle)
|
||||
}
|
||||
```
|
||||
|
||||
where `idle` class is a low task priority and tasks can be run only when the processor doesn't have anything to run besides this tasks. The second function `schedule_preempt_disabled` disables preempt in `idle` tasks. And the third function `cpu_startup_entry` is defined in the [kernel/sched/idle.c](https://github.com/torvalds/linux/blob/master/kernel/sched/idle.c) and calls `cpu_idle_loop` from the [kernel/sched/idle.c](https://github.com/torvalds/linux/blob/master/kernel/sched/idle.c). The `cpu_idle_loop` function works as process with `PID = 0` and works in the background. Main purpose of the `cpu_idle_loop` is to consume the idle CPU cycles. When there is no process to run, this process starts to work. We have one process with `idle` scheduling class (we just set the `current` task to the `idle` with the call of the `init_idle_bootup_task` function), so the `idle` thread does not do useful work but just checks if there is an active task to switch to:
|
||||
where `idle` class is a low task priority and tasks can be run only when the processor doesn't have anything to run besides this tasks. The second function `schedule_preempt_disabled` disables preempt in `idle` tasks. And the third function `cpu_startup_entry` is defined in the [kernel/sched/idle.c](https://github.com/torvalds/linux/blob/master/kernel/sched/idle.c) and calls `cpu_idle_loop` from the [kernel/sched/idle.c](https://github.com/torvalds/linux/blob/master/kernel/sched/idle.c). The `cpu_idle_loop` function works as process with `PID = 0` and works in the background. Main purpose of the `cpu_idle_loop` is to consume the idle CPU cycles. When there is no process to run, this process starts to work. We have one process with `idle` scheduling class (we just set the `current` task to the `idle` with the call of the `init_idle_bootup_task` function), so the `idle` thread does not do useful work but just checks if there is an active task to switch to:
|
||||
|
||||
```C
|
||||
static void cpu_idle_loop(void)
|
||||
@ -418,7 +418,7 @@ The `do_execve` function is defined in the [include/linux/sched.h](https://githu
|
||||
}
|
||||
```
|
||||
|
||||
If we did not pass `init=` kernel command line parameter either, kernel tries to run one of the following executable files:
|
||||
If we did not pass `init=` kernel command line parameter either, kernel tries to run one of the following executable files:
|
||||
|
||||
```C
|
||||
if (!try_to_run_init_process("/sbin/init") ||
|
||||
|
||||
@ -181,7 +181,7 @@ if (paravirt_enabled())
|
||||
return;
|
||||
```
|
||||
|
||||
we exit from the `reserve_ebda_region` function if paravirtualization is enabled because if it enabled the extended bios data area is absent. In the next step we need to get the end of the low memory:
|
||||
we exit from the `reserve_ebda_region` function if paravirtualization is enabled because if it enabled the extended BIOS data area is absent. In the next step we need to get the end of the low memory:
|
||||
|
||||
```C
|
||||
lowmem = *(unsigned short *)__va(BIOS_LOWMEM_KILOBYTES);
|
||||
@ -205,7 +205,7 @@ static inline unsigned int get_bios_ebda(void)
|
||||
}
|
||||
```
|
||||
|
||||
Let's try to understand how it works. Here we can see that we converting physical address `0x40E` to the virtual, where `0x0040:0x000e` is the segment which contains base address of the extended BIOS data area. Don't worry that we are using `phys_to_virt` function for converting a physical address to virtual address. You can note that previously we have used `__va` macro for the same point, but `phys_to_virt` is the same:
|
||||
Let's try to understand how it works. Here we can see that we are converting physical address `0x40E` to the virtual, where `0x0040:0x000e` is the segment which contains base address of the extended BIOS data area. Don't worry that we are using `phys_to_virt` function for converting a physical address to virtual address. You can note that previously we have used `__va` macro for the same point, but `phys_to_virt` is the same:
|
||||
|
||||
```C
|
||||
static inline void *phys_to_virt(phys_addr_t address)
|
||||
@ -242,7 +242,7 @@ which is:
|
||||
#define INSANE_CUTOFF 0x20000U
|
||||
```
|
||||
|
||||
or 128 kilobytes. In the last step we get lower part in the low memory and extended bios data area and call `memblock_reserve` function which will reserve memory region for extended bios data between low memory and one megabyte mark:
|
||||
or 128 kilobytes. In the last step we get lower part in the low memory and extended BIOS data area and call `memblock_reserve` function which will reserve memory region for extended BIOS data between low memory and one megabyte mark:
|
||||
|
||||
```C
|
||||
lowmem = min(lowmem, ebda_addr);
|
||||
@ -260,7 +260,7 @@ and reserves memory region for the given base address and size. `memblock_reserv
|
||||
First touch of the Linux kernel memory manager framework
|
||||
--------------------------------------------------------------------------------
|
||||
|
||||
In the previous paragraph we stopped at the call of the `memblock_reserve` function and as i said before it is the first function from the memory manager framework. Let's try to understand how it works. `memblock_reserve` function just calls:
|
||||
In the previous paragraph we stopped at the call of the `memblock_reserve` function and as I said before it is the first function from the memory manager framework. Let's try to understand how it works. `memblock_reserve` function just calls:
|
||||
|
||||
```C
|
||||
memblock_reserve_region(base, size, MAX_NUMNODES, 0);
|
||||
@ -290,7 +290,7 @@ struct memblock_type {
|
||||
};
|
||||
```
|
||||
|
||||
As we need to reserve memory block for extended bios data area, the type of the current memory region is reserved where `memblock` structure is:
|
||||
As we need to reserve memory block for extended BIOS data area, the type of the current memory region is reserved where `memblock` structure is:
|
||||
|
||||
```C
|
||||
struct memblock {
|
||||
@ -401,7 +401,7 @@ static inline void memblock_set_region_node(struct memblock_region *r, int nid)
|
||||
}
|
||||
```
|
||||
|
||||
After this we will have first reserved `memblock` for the extended bios data area in the `.meminit.data` section. `reserve_ebda_region` function finished its work on this step and we can go back to the [arch/x86/kernel/head64.c](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/arch/x86/kernel/head64.c).
|
||||
After this we will have first reserved `memblock` for the extended BIOS data area in the `.meminit.data` section. `reserve_ebda_region` function finished its work on this step and we can go back to the [arch/x86/kernel/head64.c](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/arch/x86/kernel/head64.c).
|
||||
|
||||
We finished all preparations before the kernel entry point! The last step in the `x86_64_start_reservations` function is the call of the:
|
||||
|
||||
|
||||
@ -4,7 +4,7 @@ Kernel initialization. Part 4.
|
||||
Kernel entry point
|
||||
================================================================================
|
||||
|
||||
If you have read the previous part - [Last preparations before the kernel entry point](https://github.com/0xAX/linux-insides/blob/master/Initialization/linux-initialization-3.md), you can remember that we finished all pre-initialization stuff and stopped right before the call to the `start_kernel` function from the [init/main.c](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/init/main.c). The `start_kernel` is the entry of the generic and architecture independent kernel code, although we will return to the `arch/` folder many times. If you look inside of the `start_kernel` function, you will see that this function is very big. For this moment it contains about `86` calls of functions. Yes, it's very big and of course this part will not cover all the processes that occur in this function. In the current part we will only start to do it. This part and all the next which will be in the [Kernel initialization process](https://github.com/0xAX/linux-insides/blob/master/Initialization/README.md) chapter will cover it.
|
||||
If you have read the previous part - [Last preparations before the kernel entry point](https://github.com/0xAX/linux-insides/blob/master/Initialization/linux-initialization-3.md), you can remember that we finished all pre-initialization stuff and stopped right before the call to the `start_kernel` function from the [init/main.c](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/init/main.c). The `start_kernel` is the entry of the generic and architecture independent kernel code, although we will return to the `arch/` folder many times. If you look inside of the `start_kernel` function, you will see that this function is very big. For this moment it contains about `86` function calls. Yes, it's very big and of course this part will not cover all the processes that occur in this function. In the current part we will only start to do it. This part and all the next which will be in the [Kernel initialization process](https://github.com/0xAX/linux-insides/blob/master/Initialization/README.md) chapter will cover it.
|
||||
|
||||
The main purpose of the `start_kernel` to finish kernel initialization process and launch the first `init` process. Before the first process will be started, the `start_kernel` must do many things such as: to enable [lock validator](https://www.kernel.org/doc/Documentation/locking/lockdep-design.txt), to initialize processor id, to enable early [cgroups](http://en.wikipedia.org/wiki/Cgroups) subsystem, to setup per-cpu areas, to initialize different caches in [vfs](http://en.wikipedia.org/wiki/Virtual_file_system), to initialize memory manager, rcu, vmalloc, scheduler, IRQs, ACPI and many many more. Only after these steps will we see the launch of the first `init` process in the last part of this chapter. So much kernel code awaits us, let's start.
|
||||
|
||||
@ -63,7 +63,7 @@ You can see the definition of the `init_task` and it initialized by the `INIT_TA
|
||||
|
||||
```C
|
||||
union thread_union {
|
||||
struct thread_info thread_info;
|
||||
struct thread_info thread_info;
|
||||
unsigned long stack[THREAD_SIZE/sizeof(long)];
|
||||
};
|
||||
```
|
||||
@ -74,7 +74,7 @@ Every process has its own stack and it is 16 kilobytes or 4 page frames in `x86_
|
||||
struct thread_info {
|
||||
struct task_struct *task;
|
||||
struct exec_domain *exec_domain;
|
||||
__u32 flags;
|
||||
__u32 flags;
|
||||
__u32 status;
|
||||
__u32 cpu;
|
||||
int saved_preempt_count;
|
||||
@ -179,7 +179,7 @@ As we got the end of the `init` process stack, we write `STACK_END_MAGIC` there.
|
||||
if (*end_of_stack(task) != STACK_END_MAGIC) {
|
||||
//
|
||||
// handle stack overflow here
|
||||
//
|
||||
//
|
||||
}
|
||||
```
|
||||
|
||||
@ -263,7 +263,7 @@ Remember that we have passed `cpu_number` as `pcp` to the `this_cpu_read` from t
|
||||
__bad_size_call_parameter(); break; \
|
||||
} \
|
||||
pscr_ret__; \
|
||||
})
|
||||
})
|
||||
```
|
||||
|
||||
Yes, it looks a little strange but it's easy. First of all we can see the definition of the `pscr_ret__` variable with the `int` type. Why int? Ok, `variable` is `common_cpu` and it was declared as per-cpu int variable:
|
||||
@ -272,7 +272,7 @@ Yes, it looks a little strange but it's easy. First of all we can see the defini
|
||||
DECLARE_PER_CPU_READ_MOSTLY(int, cpu_number);
|
||||
```
|
||||
|
||||
In the next step we call `__verify_pcpu_ptr` with the address of `cpu_number`. `__veryf_pcpu_ptr` used to verify that the given parameter is a per-cpu pointer. After that we set `pscr_ret__` value which depends on the size of the variable. Our `common_cpu` variable is `int`, so it 4 bytes in size. It means that we will get `this_cpu_read_4(common_cpu)` in `pscr_ret__`. In the end of the `__pcpu_size_call_return` we just call it. `this_cpu_read_4` is a macro:
|
||||
In the next step we call `__verify_pcpu_ptr` with the address of `cpu_number`. `__veryf_pcpu_ptr` used to verify that the given parameter is a per-cpu pointer. After that we set `pscr_ret__` value which depends on the size of the variable. Our `common_cpu` variable is `int`, so it's 4 bytes in size. It means that we will get `this_cpu_read_4(common_cpu)` in `pscr_ret__`. In the end of the `__pcpu_size_call_return` we just call it. `this_cpu_read_4` is a macro:
|
||||
|
||||
```C
|
||||
#define this_cpu_read_4(pcp) percpu_from_op("mov", pcp)
|
||||
|
||||
@ -165,7 +165,7 @@ The next step is initialization of early `ioremap`. In general there are two way
|
||||
|
||||
We already saw first method (`outb/inb` instructions) in the part about Linux kernel booting [process](https://0xax.gitbook.io/linux-insides/summary/booting/linux-bootstrap-3). The second method is to map I/O physical addresses to virtual addresses. When a physical address is accessed by the CPU, it may refer to a portion of physical RAM which can be mapped on memory of the I/O device. So `ioremap` used to map device memory into kernel address space.
|
||||
|
||||
As i wrote above next function is the `early_ioremap_init` which re-maps I/O memory to kernel address space so it can access it. We need to initialize early ioremap for early initialization code which needs to temporarily map I/O or memory regions before the normal mapping functions like `ioremap` are available. Implementation of this function is in the [arch/x86/mm/ioremap.c](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/arch/x86/mm/ioremap.c). At the start of the `early_ioremap_init` we can see definition of the `pmd` pointer with `pmd_t` type (which presents page middle directory entry `typedef struct { pmdval_t pmd; } pmd_t;` where `pmdval_t` is `unsigned long`) and make a check that `fixmap` aligned in a correct way:
|
||||
As I wrote above next function is the `early_ioremap_init` which re-maps I/O memory to kernel address space so it can access it. We need to initialize early ioremap for early initialization code which needs to temporarily map I/O or memory regions before the normal mapping functions like `ioremap` are available. Implementation of this function is in the [arch/x86/mm/ioremap.c](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/arch/x86/mm/ioremap.c). At the start of the `early_ioremap_init` we can see definition of the `pmd` pointer with `pmd_t` type (which presents page middle directory entry `typedef struct { pmdval_t pmd; } pmd_t;` where `pmdval_t` is `unsigned long`) and make a check that `fixmap` aligned in a correct way:
|
||||
|
||||
```C
|
||||
pmd_t *pmd;
|
||||
@ -298,7 +298,7 @@ presents abstraction for a tree-like subset of system resources. This structure
|
||||
|
|
||||
+-------------+
|
||||
| |
|
||||
| child |
|
||||
| child |
|
||||
| |
|
||||
+-------------+
|
||||
```
|
||||
@ -422,7 +422,7 @@ The next step is initialization of the memory descriptor of the init process. As
|
||||
}
|
||||
```
|
||||
|
||||
`mm` points to the process address space and `active_mm` points to the active address space if process has no address space such as kernel threads (more about it you can read in the [documentation](https://www.kernel.org/doc/Documentation/vm/active_mm.txt)). Now we fill memory descriptor of the initial process:
|
||||
`mm` points to the process address space and `active_mm` points to the active address space if process has no address space such as kernel threads (more about it you can read in the [documentation](https://www.kernel.org/doc/Documentation/vm/active_mm.txt)). Now we fill memory descriptor of the initial process:
|
||||
|
||||
```C
|
||||
init_mm.start_code = (unsigned long) _text;
|
||||
@ -457,7 +457,7 @@ where `mm_rb` is a red-black tree of the virtual memory areas, `pgd` is a pointe
|
||||
bss_resource.end = __pa_symbol(__bss_stop)-1;
|
||||
```
|
||||
|
||||
We already know a little about `resource` structure (read above). Here we fills code/data/bss resources with their physical addresses. You can see it in the `/proc/iomem`:
|
||||
We already know a little about `resource` structure (read above). Here we fill code/data/bss resources with their physical addresses. You can see it in the `/proc/iomem`:
|
||||
|
||||
```C
|
||||
00100000-be825fff : System RAM
|
||||
@ -477,7 +477,7 @@ static struct resource code_resource = {
|
||||
};
|
||||
```
|
||||
|
||||
The last step which we will cover in this part will be `NX` configuration. `NX-bit` or no execute bit is 63-bit in the page directory entry which controls the ability to execute code from all physical pages mapped by the table entry. This bit can only be used/set when the `no-execute` page-protection mechanism is enabled by the setting `EFER.NXE` to 1. In the `x86_configure_nx` function we check that CPU has support of `NX-bit` and it does not disabled. After the check we fill `__supported_pte_mask` depend on it:
|
||||
The last step which we will cover in this part will be `NX` configuration. `NX-bit` or no execute bit is 63-bit in the page directory entry which controls the ability to execute code from all physical pages mapped by the table entry. This bit can only be used/set when the `no-execute` page-protection mechanism is enabled by the setting `EFER.NXE` to 1. In the `x86_configure_nx` function we check that CPU has support of `NX-bit` and it does not disabled. After the check we fill `__supported_pte_mask` depend on it:
|
||||
|
||||
```C
|
||||
void x86_configure_nx(void)
|
||||
@ -492,7 +492,7 @@ void x86_configure_nx(void)
|
||||
Conclusion
|
||||
--------------------------------------------------------------------------------
|
||||
|
||||
It is the end of the fifth part about Linux kernel initialization process. In this part we continued to dive in the `setup_arch` function which makes initialization of architecture-specific stuff. It was long part, but we have not finished with it. As i already wrote, the `setup_arch` is big function, and I am really not sure that we will cover all of it even in the next part. There were some new interesting concepts in this part like `Fix-mapped` addresses, ioremap and etc... Don't worry if they are unclear for you. There is a special part about these concepts - [Linux kernel memory management Part 2.](https://github.com/0xAX/linux-insides/blob/master/MM/linux-mm-2.md). In the next part we will continue with the initialization of the architecture-specific stuff and will see parsing of the early kernel parameters, early dump of the pci devices, `Desktop Management Interface` scanning and many many more.
|
||||
It is the end of the fifth part about Linux kernel initialization process. In this part we continued to dive in the `setup_arch` function which makes initialization of architecture-specific stuff. It was long part, but we are not finished with it. As I already wrote, the `setup_arch` is big function, and I am really not sure that we will cover all of it even in the next part. There were some new interesting concepts in this part like `Fix-mapped` addresses, ioremap and etc... Don't worry if they are unclear for you. There is a special part about these concepts - [Linux kernel memory management Part 2.](https://github.com/0xAX/linux-insides/blob/master/MM/linux-mm-2.md). In the next part we will continue with the initialization of the architecture-specific stuff and will see parsing of the early kernel parameters, early dump of the pci devices, `Desktop Management Interface` scanning and many many more.
|
||||
|
||||
If you have any questions or suggestions write me a comment or ping me at [twitter](https://twitter.com/0xAX).
|
||||
|
||||
|
||||
@ -60,7 +60,7 @@ Note that `__set_param` macro defines with `__section(.init.setup)` attribute. I
|
||||
VMLINUX_SYMBOL(__setup_end) = .;
|
||||
```
|
||||
|
||||
Now we know how parameters are defined, let's back to the `parse_early_param` implementation:
|
||||
Now we know how parameters are defined, let's back to the `parse_early_param` implementation:
|
||||
|
||||
```C
|
||||
void __init parse_early_param(void)
|
||||
@ -128,7 +128,7 @@ int __init acpi_mps_check(void)
|
||||
}
|
||||
```
|
||||
|
||||
It checks the built-in `MPS` or [MultiProcessor Specification](http://en.wikipedia.org/wiki/MultiProcessor_Specification) table. If `CONFIG_X86_LOCAL_APIC` is set and `CONFIG_x86_MPPAARSE` is not set, `acpi_mps_check` prints warning message if the one of the command line options: `acpi=off`, `acpi=noirq` or `pci=noacpi` passed to the kernel. If `acpi_mps_check` returns `1` it means that we disable local [APIC](http://en.wikipedia.org/wiki/Advanced_Programmable_Interrupt_Controller) and clear `X86_FEATURE_APIC` bit in the of the current CPU with the `setup_clear_cpu_cap` macro. (more about CPU mask you can read in the [CPU masks](https://0xax.gitbook.io/linux-insides/summary/concepts/linux-cpu-2)).
|
||||
It checks the built-in `MPS` or [MultiProcessor Specification](http://en.wikipedia.org/wiki/MultiProcessor_Specification) table. If `CONFIG_X86_LOCAL_APIC` is set and `CONFIG_x86_MPPARSE` is not set, `acpi_mps_check` prints warning message if the one of the command line options: `acpi=off`, `acpi=noirq` or `pci=noacpi` passed to the kernel. If `acpi_mps_check` returns `1` it means that we disable local [APIC](http://en.wikipedia.org/wiki/Advanced_Programmable_Interrupt_Controller) and clear `X86_FEATURE_APIC` bit in the of the current CPU with the `setup_clear_cpu_cap` macro. (more about CPU mask you can read in the [CPU masks](https://0xax.gitbook.io/linux-insides/summary/concepts/linux-cpu-2)).
|
||||
|
||||
Early PCI dump
|
||||
--------------------------------------------------------------------------------
|
||||
@ -165,7 +165,7 @@ char *__init pcibios_setup(char *str) {
|
||||
}
|
||||
```
|
||||
|
||||
So, if `CONFIG_PCI` option is set and we passed `pci=earlydump` option to the kernel command line, next function which will be called - `early_dump_pci_devices` from the [arch/x86/pci/early.c](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/arch/x86/pci/early.c). This function checks `noearly` pci parameter with:
|
||||
So, if `CONFIG_PCI` option is set and we passed `pci=earlydump` option to the kernel command line, next function which will be called - `early_dump_pci_devices` from the [arch/x86/pci/early.c](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/arch/x86/pci/early.c). This function checks `noearly` PCI parameter with:
|
||||
|
||||
```C
|
||||
if (!early_pci_allowed())
|
||||
@ -208,7 +208,7 @@ After the `early_dump_pci_devices`, there are a couple of function related with
|
||||
early_reserve_e820_mpc_new();
|
||||
```
|
||||
|
||||
Let's look on it. As you can see the first function is `e820_reserve_setup_data`. This function does almost the same as `memblock_x86_reserve_range_setup_data` which we saw above, but it also calls `e820_update_range` which adds new regions to the `e820map` with the given type which is `E820_RESERVED_KERN` in our case. The next function is `finish_e820_parsing` which sanitizes `e820map` with the `sanitize_e820_map` function. Besides this two functions we can see a couple of functions related to the [e820](http://en.wikipedia.org/wiki/E820). You can see it in the listing above. `e820_add_kernel_range` function takes the physical address of the kernel start and end:
|
||||
Let's look at it. As you can see the first function is `e820_reserve_setup_data`. This function does almost the same as `memblock_x86_reserve_range_setup_data` which we saw above, but it also calls `e820_update_range` which adds new regions to the `e820map` with the given type which is `E820_RESERVED_KERN` in our case. The next function is `finish_e820_parsing` which sanitizes `e820map` with the `sanitize_e820_map` function. Besides this two functions we can see a couple of functions related to the [e820](http://en.wikipedia.org/wiki/E820). You can see it in the listing above. `e820_add_kernel_range` function takes the physical address of the kernel start and end:
|
||||
|
||||
```C
|
||||
u64 start = __pa_symbol(_text);
|
||||
@ -273,13 +273,13 @@ if (max_pfn > (1UL<<(32 - PAGE_SHIFT)))
|
||||
max_low_pfn = e820_end_of_low_ram_pfn();
|
||||
else
|
||||
max_low_pfn = max_pfn;
|
||||
|
||||
|
||||
high_memory = (void *)__va(max_pfn * PAGE_SIZE - 1) + 1;
|
||||
```
|
||||
|
||||
Next we calculate `high_memory` (defines the upper bound on direct map memory) with `__va` macro which returns a virtual address by the given physical memory.
|
||||
|
||||
DMI scanning
|
||||
DMI scanning
|
||||
-------------------------------------------------------------------------------
|
||||
|
||||
The next step after manipulations with different memory regions and `e820` slots is collecting information about computer. We will get all information with the [Desktop Management Interface](http://en.wikipedia.org/wiki/Desktop_Management_Interface) and following functions:
|
||||
@ -356,7 +356,7 @@ RESERVE_BRK(dmi_alloc, 65536);
|
||||
#endif
|
||||
```
|
||||
|
||||
`RESERVE_BRK` defined in the [arch/x86/include/asm/setup.h](http://en.wikipedia.org/wiki/Desktop_Management_Interface) and reserves space with given size in the `brk` section.
|
||||
`RESERVE_BRK` defined in the [arch/x86/include/asm/setup.h](http://github.com/torvalds/linux/blob/master/arch/x86/include/asm/setup.h) and reserves space with given size in the `brk` section.
|
||||
|
||||
-------------------------
|
||||
init_hypervisor_platform();
|
||||
|
||||
@ -19,7 +19,7 @@ Now let's look on the implementation of the `setup_log_buf` function. It starts
|
||||
```C
|
||||
if (log_buf != __log_buf)
|
||||
return;
|
||||
|
||||
|
||||
if (!early && !new_log_buf_len)
|
||||
log_buf_add_cpu();
|
||||
```
|
||||
@ -41,7 +41,7 @@ if (ramdisk_size >= (mapped_size>>1))
|
||||
"disabling initrd (%lld needed, %lld available)\n",
|
||||
ramdisk_size, mapped_size>>1);
|
||||
```
|
||||
|
||||
|
||||
You can see here that we call `memblock_mem_size` function and pass the `max_pfn_mapped` to it, where `max_pfn_mapped` contains the highest direct mapped page frame number. If you do not remember what is `page frame number`, explanation is simple: First `12` bits of the virtual address represent offset in the physical page or page frame. If we right-shift out `12` bits of the virtual address, we'll discard offset part and will get `Page Frame Number`. In the `memblock_mem_size` we go through the all memblock `mem` (not reserved) regions and calculates size of the mapped pages and return it to the `mapped_size` variable (see code above). As we got amount of the direct mapped memory, we check that size of the `initrd` is not greater than mapped pages. If it is greater we just call `panic` which halts the system and prints famous [Kernel panic](http://en.wikipedia.org/wiki/Kernel_panic) message. In the next step we print information about the `initrd` size. We can see the result of this in the `dmesg` output:
|
||||
|
||||
```C
|
||||
@ -189,7 +189,7 @@ The next step is the call of the function - `x86_init.paging.pagetable_init`. If
|
||||
#define native_pagetable_init paging_init
|
||||
```
|
||||
|
||||
which expands as you can see to the call of the `paging_init` function from the [arch/x86/mm/init_64.c](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/arch/x86/mm/init_64.c). The `paging_init` function initializes sparse memory and zone sizes. First of all what's zones and what is it `Sparsemem`. The `Sparsemem` is a special foundation in the Linux kernel memory manager which used to split memory area into different memory banks in the [NUMA](http://en.wikipedia.org/wiki/Non-uniform_memory_access) systems. Let's look on the implementation of the `paginig_init` function:
|
||||
which expands as you can see to the call of the `paging_init` function from the [arch/x86/mm/init_64.c](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/arch/x86/mm/init_64.c). The `paging_init` function initializes sparse memory and zone sizes. First of all what's zones and what is it `Sparsemem`. The `Sparsemem` is a special foundation in the Linux kernel memory manager which used to split memory area into different memory banks in the [NUMA](http://en.wikipedia.org/wiki/Non-uniform_memory_access) systems. Let's look on the implementation of the `paging_init` function:
|
||||
|
||||
```C
|
||||
void __init paging_init(void)
|
||||
@ -209,7 +209,7 @@ As you can see there is call of the `sparse_memory_present_with_active_regions`
|
||||
|
||||
Again, this part and next parts do not cover this theme in full details. There will be special part about `NUMA`.
|
||||
|
||||
vsyscall mapping
|
||||
vsyscall mapping
|
||||
--------------------------------------------------------------------------------
|
||||
|
||||
The next step after `SparseMem` initialization is setting of the `trampoline_cr4_features` which must contain content of the `cr4` [Control register](http://en.wikipedia.org/wiki/Control_register). First of all we need to check that current CPU has support of the `cr4` register and if it has, we save its content to the `trampoline_cr4_features` which is storage for `cr4` in the real mode:
|
||||
@ -315,7 +315,7 @@ struct mpf_intel *mpf = mpf_found;
|
||||
|
||||
if (!mpf)
|
||||
return;
|
||||
|
||||
|
||||
if (acpi_lapic && early)
|
||||
return;
|
||||
```
|
||||
@ -325,7 +325,7 @@ Here we can see that multiprocessor configuration was found in the `smp_scan_con
|
||||
The rest of the setup_arch
|
||||
--------------------------------------------------------------------------------
|
||||
|
||||
Here we are getting to the end of the `setup_arch` function. The rest of function of course is important, but details about these stuff will not will not be included in this part. We will just take a short look on these functions, because although they are important as I wrote above, but they cover non-generic kernel features related with the `NUMA`, `SMP`, `ACPI` and `APICs`, etc. First of all, the next call of the `init_apic_mappings` function. As we can understand this function sets the address of the local [APIC](http://en.wikipedia.org/wiki/Advanced_Programmable_Interrupt_Controller). The next is `x86_io_apic_ops.init` and this function initializes I/O APIC. Please note that we will see all details related with `APIC` in the chapter about interrupts and exceptions handling. In the next step we reserve standard I/O resources like `DMA`, `TIMER`, `FPU`, etc., with the call of the `x86_init.resources.reserve_resources` function. Following is `mcheck_init` function initializes `Machine check Exception` and the last is `register_refined_jiffies` which registers [jiffy](http://en.wikipedia.org/wiki/Jiffy_%28time%29) (There will be separate chapter about timers in the kernel).
|
||||
Here we are getting to the end of the `setup_arch` function. The rest of function of course is important, but details about these stuff will not will not be included in this part. We will just take a short look on these functions, because although they are important as I wrote above, they cover non-generic kernel features related with the `NUMA`, `SMP`, `ACPI` and `APICs`, etc. First of all, the next call of the `init_apic_mappings` function. As we can understand this function sets the address of the local [APIC](http://en.wikipedia.org/wiki/Advanced_Programmable_Interrupt_Controller). The next is `x86_io_apic_ops.init` and this function initializes I/O APIC. Please note that we will see all details related with `APIC` in the chapter about interrupts and exceptions handling. In the next step we reserve standard I/O resources like `DMA`, `TIMER`, `FPU`, etc., with the call of the `x86_init.resources.reserve_resources` function. Following is `mcheck_init` function initializes `Machine check Exception` and the last is `register_refined_jiffies` which registers [jiffy](http://en.wikipedia.org/wiki/Jiffy_%28time%29) (There will be separate chapter about timers in the kernel).
|
||||
|
||||
So that's all. Finally we have finished with the big `setup_arch` function in this part. Of course as I already wrote many times, we did not see full details about this function, but do not worry about it. We will be back more than once to this function from different chapters for understanding how different platform-dependent parts are initialized.
|
||||
|
||||
|
||||
@ -75,7 +75,7 @@ static inline struct desc_struct *get_cpu_gdt_table(unsigned int cpu)
|
||||
|
||||
The `get_cpu_gdt_table` uses `per_cpu` macro for getting value of a `gdt_page` percpu variable for the given CPU number (bootstrap processor with `id` - 0 in our case).
|
||||
|
||||
You may ask the following question: so, if we can access `gdt_page` percpu variable, where it was defined? Actually we already saw it in this book. If you have read the first [part](https://0xax.gitbook.io/linux-insides/summary/initialization/linux-initialization-1) of this chapter, you can remember that we saw definition of the `gdt_page` in the [arch/x86/kernel/head_64.S](https://github.com/0xAX/linux/blob/0a07b238e5f488b459b6113a62e06b6aab017f71/arch/x86/kernel/head_64.S):
|
||||
You may ask the following question: so, if we can access `gdt_page` percpu variable, where was it defined? Actually we already saw it in this book. If you have read the first [part](https://0xax.gitbook.io/linux-insides/summary/initialization/linux-initialization-1) of this chapter, you can remember that we saw definition of the `gdt_page` in the [arch/x86/kernel/head_64.S](https://github.com/0xAX/linux/blob/0a07b238e5f488b459b6113a62e06b6aab017f71/arch/x86/kernel/head_64.S):
|
||||
|
||||
```assembly
|
||||
early_gdt_descr:
|
||||
@ -117,29 +117,29 @@ void load_percpu_segment(int cpu) {
|
||||
}
|
||||
```
|
||||
|
||||
The base address of the `percpu` area must contain `gs` register (or `fs` register for `x86`), so we are using `loadsegment` macro and pass `gs`. In the next step we writes the base address if the [IRQ](http://en.wikipedia.org/wiki/Interrupt_request_%28PC_architecture%29) stack and setup stack [canary](http://en.wikipedia.org/wiki/Buffer_overflow_protection) (this is only for `x86_32`). After we load new `GDT`, we fill `cpu_callout_mask` bitmap with the current cpu and set cpu state as online with the setting `cpu_state` percpu variable for the current processor - `CPU_ONLINE`:
|
||||
The base address of the `percpu` area must contain `gs` register (or `fs` register for `x86`), so we are using `loadsegment` macro and pass `gs`. In the next step we write the base address if the [IRQ](http://en.wikipedia.org/wiki/Interrupt_request_%28PC_architecture%29) stack and setup stack [canary](http://en.wikipedia.org/wiki/Buffer_overflow_protection) (this is only for `x86_32`). After we load new `GDT`, we fill `cpu_callout_mask` bitmap with the current cpu and set cpu state as online with the setting `cpu_state` percpu variable for the current processor - `CPU_ONLINE`:
|
||||
|
||||
```C
|
||||
cpumask_set_cpu(me, cpu_callout_mask);
|
||||
per_cpu(cpu_state, me) = CPU_ONLINE;
|
||||
```
|
||||
|
||||
So, what is `cpu_callout_mask` bitmap... As we initialized bootstrap processor (processor which is booted the first on `x86`) the other processors in a multiprocessor system are known as `secondary processors`. Linux kernel uses following two bitmasks:
|
||||
So, what is `cpu_callout_mask` bitmap? As we initialized bootstrap processor (processor which is booted the first on `x86`) the other processors in a multiprocessor system are known as `secondary processors`. Linux kernel uses following two bitmasks:
|
||||
|
||||
* `cpu_callout_mask`
|
||||
* `cpu_callin_mask`
|
||||
|
||||
After bootstrap processor initialized, it updates the `cpu_callout_mask` to indicate which secondary processor can be initialized next. All other or secondary processors can do some initialization stuff before and check the `cpu_callout_mask` on the bootstrap processor bit. Only after the bootstrap processor filled the `cpu_callout_mask` with this secondary processor, it will continue the rest of its initialization. After that the certain processor finish its initialization process, the processor sets bit in the `cpu_callin_mask`. Once the bootstrap processor finds the bit in the `cpu_callin_mask` for the current secondary processor, this processor repeats the same procedure for initialization of one of the remaining secondary processors. In a short words it works as i described, but we will see more details in the chapter about `SMP`.
|
||||
|
||||
After bootstrap processor initialized, it updates the `cpu_callout_mask` to indicate which secondary processor can be initialized next. All other or secondary processors can do some initialization stuff before and check the `cpu_callout_mask` on the bootstrap processor bit. Only after the bootstrap processor filled the `cpu_callout_mask` with this secondary processor, it will continue the rest of its initialization. After that the certain processor finish its initialization process, the processor sets bit in the `cpu_callin_mask`. Once the bootstrap processor finds the bit in the `cpu_callin_mask` for the current secondary processor, this processor repeats the same procedure for initialization of one of the remaining secondary processors. In a short words it works as I described, but we will see more details in the chapter about `SMP`.
|
||||
|
||||
That's all. We did all `SMP` boot preparation.
|
||||
|
||||
Build zonelists
|
||||
-----------------------------------------------------------------------
|
||||
|
||||
In the next step we can see the call of the `build_all_zonelists` function. This function sets up the order of zones that allocations are preferred from. What are zones and what's order we will understand soon. For the start let's see how Linux kernel considers physical memory. Physical memory is split into banks which are called - `nodes`. If you has no hardware support for `NUMA`, you will see only one node:
|
||||
In the next step we can see the call of the `build_all_zonelists` function. This function sets up the order of zones that allocations are preferred from. What are zones and what's order we will understand soon. For the start let's see how Linux kernel considers physical memory. Physical memory is split into banks which are called - `nodes`. If you have no hardware support for `NUMA`, you will see only one node:
|
||||
|
||||
```
|
||||
$ cat /sys/devices/system/node/node0/numastat
|
||||
$ cat /sys/devices/system/node/node0/numastat
|
||||
numa_hit 72452442
|
||||
numa_miss 0
|
||||
numa_foreign 0
|
||||
@ -185,13 +185,13 @@ As I wrote above all nodes are described with the `pglist_data` or `pg_data_t` s
|
||||
The rest of the stuff before scheduler initialization
|
||||
--------------------------------------------------------------------------------
|
||||
|
||||
Before we will start to dive into Linux kernel scheduler initialization process we must do a couple of things. The first thing is the `page_alloc_init` function from the [mm/page_alloc.c](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/mm/page_alloc.c). This function looks pretty easy:
|
||||
Before we start to dive into Linux kernel scheduler initialization process we must do a couple of things. The first thing is the `page_alloc_init` function from the [mm/page_alloc.c](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/mm/page_alloc.c). This function looks pretty easy:
|
||||
|
||||
```C
|
||||
void __init page_alloc_init(void)
|
||||
{
|
||||
int ret;
|
||||
|
||||
|
||||
ret = cpuhp_setup_state_nocalls(CPUHP_PAGE_ALLOC_DEAD,
|
||||
"mm/page_alloc:dead", NULL,
|
||||
page_alloc_cpu_dead);
|
||||
@ -230,7 +230,7 @@ pid_hash = alloc_large_system_hash("PID", sizeof(*pid_hash), 0, 18,
|
||||
```
|
||||
|
||||
The number of elements of the `pid_hash` depends on the `RAM` configuration, but it can be between `2^4` and `2^12`. The `pidhash_init` computes the size
|
||||
and allocates the required storage (which is `hlist` in our case - the same as [doubly linked list](https://0xax.gitbook.io/linux-insides/summary/datastructures/linux-datastructures-1), but contains one pointer instead on the [struct hlist_head](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/include/linux/types.h)]. The `alloc_large_system_hash` function allocates a large system hash table with `memblock_virt_alloc_nopanic` if we pass `HASH_EARLY` flag (as it in our case) or with `__vmalloc` if we did no pass this flag.
|
||||
and allocates the required storage (which is `hlist` in our case - the same as [doubly linked list](https://0xax.gitbook.io/linux-insides/summary/datastructures/linux-datastructures-1), but contains one pointer instead on the [struct hlist_head](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/include/linux/types.h). The `alloc_large_system_hash` function allocates a large system hash table with `memblock_virt_alloc_nopanic` if we pass `HASH_EARLY` flag (as it in our case) or with `__vmalloc` if we did no pass this flag.
|
||||
|
||||
The result we can see in the `dmesg` output:
|
||||
|
||||
@ -255,7 +255,7 @@ pgtable_init();
|
||||
vmalloc_init();
|
||||
```
|
||||
|
||||
The first is `page_ext_init_flatmem` which depends on the `CONFIG_SPARSEMEM` kernel configuration option and initializes extended data per page handling. The `mem_init` releases all `bootmem`, the `kmem_cache_init` initializes kernel cache, the `percpu_init_late` - replaces `percpu` chunks with those allocated by [slub](http://en.wikipedia.org/wiki/SLUB_%28software%29), the `pgtable_init` - initializes the `page->ptl` kernel cache, the `vmalloc_init` - initializes `vmalloc`. Please, **NOTE** that we will not dive into details about all of these functions and concepts, but we will see all of they it in the [Linux kernel memory manager](https://0xax.gitbook.io/linux-insides/summary/mm) chapter.
|
||||
The first is `page_ext_init_flatmem` which depends on the `CONFIG_SPARSEMEM` kernel configuration option and initializes extended data per page handling. The `mem_init` releases all `bootmem`, the `kmem_cache_init` initializes kernel cache, the `percpu_init_late` - replaces `percpu` chunks with those allocated by [slub](http://en.wikipedia.org/wiki/SLUB_%28software%29), the `pgtable_init` - initializes the `page->ptl` kernel cache, the `vmalloc_init` - initializes `vmalloc`. Please, **NOTE** that we will not dive into details about all of these functions and concepts, but we will see all of them it in the [Linux kernel memory manager](https://0xax.gitbook.io/linux-insides/summary/mm) chapter.
|
||||
|
||||
That's all. Now we can look on the `scheduler`.
|
||||
|
||||
@ -318,7 +318,7 @@ The `Completely Fair Scheduler` supports following `normal` or in other words `n
|
||||
|
||||
The `SCHED_NORMAL` is used for the most normal applications, the amount of cpu each process consumes is mostly determined by the [nice](http://en.wikipedia.org/wiki/Nice_%28Unix%29) value, the `SCHED_BATCH` used for the 100% non-interactive tasks and the `SCHED_IDLE` runs tasks only when the processor has no task to run besides this task.
|
||||
|
||||
The `real-time` policies are also supported for the time-critical applications: `SCHED_FIFO` and `SCHED_RR`. If you've read something about the Linux kernel scheduler, you can know that it is modular. That means it supports different algorithms to schedule different types of processes. Usually this modularity is called `scheduler classes`. These modules encapsulate scheduling policy details and are handled by the scheduler core without knowing too much about them.
|
||||
The `real-time` policies are also supported for the time-critical applications: `SCHED_FIFO` and `SCHED_RR`. If you've read something about the Linux kernel scheduler, you can know that it is modular. That means it supports different algorithms to schedule different types of processes. Usually this modularity is called `scheduler classes`. These modules encapsulate scheduling policy details and are handled by the scheduler core without knowing too much about them.
|
||||
|
||||
Now let's get back to the our code and look on the two configuration options: `CONFIG_FAIR_GROUP_SCHED` and `CONFIG_RT_GROUP_SCHED`. The smallest unit that the scheduler works with is an individual task or thread. However, a process is not the only type of entity that the scheduler can operate with. Both of these options provide support for group scheduling. The first option provides support for group scheduling with the `completely fair scheduler` policies and the second with the `real-time` policies respectively.
|
||||
|
||||
@ -340,11 +340,11 @@ The first is for case when scheduling of task groups is enabled with `completely
|
||||
* scheduler entity structure;
|
||||
* `runqueue`.
|
||||
|
||||
After we have calculated size, we allocate a space with the `kzalloc` function and set pointers of `sched_entity` and `runquques` there:
|
||||
After we have calculated size, we allocate a space with the `kzalloc` function and set pointers of `sched_entity` and `runqueues` there:
|
||||
|
||||
```C
|
||||
ptr = (unsigned long)kzalloc(alloc_size, GFP_NOWAIT);
|
||||
|
||||
|
||||
#ifdef CONFIG_FAIR_GROUP_SCHED
|
||||
root_task_group.se = (struct sched_entity **)ptr;
|
||||
ptr += nr_cpu_ids * sizeof(void **);
|
||||
@ -396,10 +396,10 @@ All groups have to be able to rely on the amount of CPU time. The two following
|
||||
The first represents a period and the second represents quantum that is allocated for `real-time` tasks during `sched_rt_period_us`. You may see global values of these parameters in the:
|
||||
|
||||
```
|
||||
$ cat /proc/sys/kernel/sched_rt_period_us
|
||||
$ cat /proc/sys/kernel/sched_rt_period_us
|
||||
1000000
|
||||
|
||||
$ cat /proc/sys/kernel/sched_rt_runtime_us
|
||||
$ cat /proc/sys/kernel/sched_rt_runtime_us
|
||||
950000
|
||||
```
|
||||
|
||||
@ -415,7 +415,7 @@ That's all with the bandwiths of `real-time` and `deadline` tasks and in the nex
|
||||
|
||||
The real-time scheduler requires global resources to make scheduling decision. But unfortunately scalability bottlenecks appear as the number of CPUs increase. The concept of `root domains` was introduced for improving scalability and avoid such bottlenecks. Instead of bypassing over all `run queues`, the scheduler gets information about a CPU where/from to push/pull a `real-time` task from the `root_domain` structure. This structure is defined in the [kernel/sched/sched.h](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/kernel/sched/sched.h) kernel header file and just keeps track of CPUs that can be used to push or pull a process.
|
||||
|
||||
After `root domain` initialization, we make initialization of the `bandwidth` for the `real-time` tasks of the `root task group` as we did the same above:
|
||||
After `root domain` initialization, we make initialization of the `bandwidth` for the `real-time` tasks of the `root task group` as we did the same above:
|
||||
```C
|
||||
#ifdef CONFIG_RT_GROUP_SCHED
|
||||
init_rt_bandwidth(&root_task_group.rt_bandwidth,
|
||||
@ -499,7 +499,7 @@ struct task_struct {
|
||||
}
|
||||
```
|
||||
|
||||
The first one is `dynamic priority` which can't be changed during lifetime of a process based on its static priority and interactivity of the process. The `static_prio` contains initial priority most likely well-known to you `nice value`. This value does not changed by the kernel if a user will not change it. The last one is `normal_priority` based on the value of the `static_prio` too, but also it depends on the scheduling policy of a process.
|
||||
The first one is `dynamic priority` which can't be changed during lifetime of a process based on its static priority and interactivity of the process. The `static_prio` contains initial priority most likely well-known to you `nice value`. This value is not changed by the kernel if a user does not change it. The last one is `normal_priority` based on the value of the `static_prio` too, but also it depends on the scheduling policy of a process.
|
||||
|
||||
So the main goal of the `set_load_weight` function is to initialize `load_weight` fields for the `init` task:
|
||||
|
||||
|
||||
@ -120,7 +120,7 @@ More about integer ID management you can read [here](https://lwn.net/Articles/10
|
||||
RCU initialization
|
||||
--------------------------------------------------------------------------------
|
||||
|
||||
The next step is [RCU](http://en.wikipedia.org/wiki/Read-copy-update) initialization with the `rcu_init` function and it's implementation depends on two kernel configuration options:
|
||||
The next step is [RCU](http://en.wikipedia.org/wiki/Read-copy-update) initialization with the `rcu_init` function and its implementation depends on two kernel configuration options:
|
||||
|
||||
* `CONFIG_TINY_RCU`
|
||||
* `CONFIG_TREE_RCU`
|
||||
@ -292,7 +292,7 @@ extern struct rcu_state rcu_bh_state;
|
||||
DECLARE_PER_CPU(struct rcu_data, rcu_bh_data);
|
||||
```
|
||||
|
||||
About this states you can read [here](http://lwn.net/Articles/264090/). As I wrote above we need to initialize `rcu_state` structures and `rcu_init_one` function will help us with it. After the `rcu_state` initialization, we can see the call of the ` __rcu_init_preempt` which depends on the `CONFIG_PREEMPT_RCU` kernel configuration option. It does the same as previous functions - initialization of the `rcu_preempt_state` structure with the `rcu_init_one` function which has `rcu_state` type. After this, in the `rcu_init`, we can see the call of the:
|
||||
About these states you can read [here](http://lwn.net/Articles/264090/). As I wrote above we need to initialize `rcu_state` structures and `rcu_init_one` function will help us with it. After the `rcu_state` initialization, we can see the call of the ` __rcu_init_preempt` which depends on the `CONFIG_PREEMPT_RCU` kernel configuration option. It does the same as previous functions - initialization of the `rcu_preempt_state` structure with the `rcu_init_one` function which has `rcu_state` type. After this, in the `rcu_init`, we can see the call of the:
|
||||
|
||||
```C
|
||||
open_softirq(RCU_SOFTIRQ, rcu_process_callbacks);
|
||||
|
||||
Binary file not shown.
|
Before Width: | Height: | Size: 66 KiB After Width: | Height: | Size: 47 KiB |
@ -281,8 +281,7 @@ The `PAGE_SIZE` is `4096`-bytes and the `THREAD_SIZE_ORDER` depends on the `KASA
|
||||
#define IRQ_STACK_SIZE (PAGE_SIZE << IRQ_STACK_ORDER)
|
||||
```
|
||||
|
||||
Or `16384` bytes. The per-cpu interrupt stack is represented by the `irq_stack` struct and the `fixed_percpu_data` struct
|
||||
in the Linux kernel for `x86_64`:
|
||||
Or `16384` bytes. The per-cpu interrupt stack is represented by the `irq_stack` struct and the `fixed_percpu_data` struct in the Linux kernel for `x86_64`:
|
||||
|
||||
```C
|
||||
/* Per CPU interrupt stacks */
|
||||
@ -306,7 +305,7 @@ struct fixed_percpu_data {
|
||||
#endif
|
||||
```
|
||||
|
||||
The `irq_stack` struct contains a 16 kilobytes array.
|
||||
The `irq_stack` struct contains a 16 kilobytes array.
|
||||
Also, you can see that the fixed\_percpu\_data contains two fields:
|
||||
|
||||
* `gs_base` - The `gs` register always points to the bottom of the `fixed_percpu_data`. On the `x86_64`, the `gs` register is shared by per-cpu area and stack canary (more about `per-cpu` variables you can read in the special [part](https://0xax.gitbook.io/linux-insides/summary/concepts/linux-cpu-1)). All per-cpu symbols are zero-based and the `gs` points to the base of the per-cpu area. You already know that [segmented memory model](http://en.wikipedia.org/wiki/Memory_segmentation) is abolished in the long mode, but we can set the base address for the two segment registers - `fs` and `gs` with the [Model specific registers](http://en.wikipedia.org/wiki/Model-specific_register) and these registers can be still be used as address registers. If you remember the first [part](https://0xax.gitbook.io/linux-insides/summary/initialization/linux-initialization-1) of the Linux kernel initialization process, you can remember that we have set the `gs` register:
|
||||
@ -373,11 +372,11 @@ int irq_init_percpu_irqstack(unsigned int cpu)
|
||||
|
||||
Here we go over all the CPUs one-by-one and setup the `hardirq_stack_ptr`.
|
||||
Where `map_irq_stack` is called to initialize the `hardirq_stack_ptr`,
|
||||
to point onto the `irq_backing_store` of the current CPU with an offset of IRQ\_STACK\_SIZE,
|
||||
to point onto the `irq_stack_backing_store` of the current CPU with an offset of IRQ\_STACK\_SIZE,
|
||||
either with guard pages or without when KASan is enabled.
|
||||
|
||||
|
||||
After the initialization of the interrupt stack, we need to initialize the gs register within [arch/x86/kernel/cpu/common.c](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/arch/x86/kernel/cpu/common.c):
|
||||
After the initialization of the interrupt stack, we need to initialize the gs register within [arch/x86/kernel/cpu/common.c](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/arch/x86/kernel/cpu/common.c):
|
||||
|
||||
```C
|
||||
void load_percpu_segment(int cpu)
|
||||
@ -406,7 +405,7 @@ and as we already know the `gs` register points to the bottom of the interrupt s
|
||||
|
||||
Here we can see the `wrmsr` instruction, which loads the data from `edx:eax` into the [Model specific register](http://en.wikipedia.org/wiki/Model-specific_register) pointed by the `ecx` register. In our case the model specific register is `MSR_GS_BASE`, which contains the base address of the memory segment pointed to by the `gs` register. `edx:eax` points to the address of the `initial_gs,` which is the base address of our `fixed_percpu_data`.
|
||||
|
||||
We already know that `x86_64` has a feature called `Interrupt Stack Table` or `IST` and this feature provides the ability to switch to a new stack for events like a non-maskable interrupt, double fault etc. There can be up to seven `IST` entries per-cpu. Some of them are:
|
||||
We already know that `x86_64` has a feature called `Interrupt Stack Table` or `IST` and this feature provides the ability to switch to a new stack for events like a non-maskable interrupt, double fault, etc. There can be up to seven `IST` entries per-cpu. Some of them are:
|
||||
|
||||
* `DOUBLEFAULT_STACK`
|
||||
* `NMI_STACK`
|
||||
@ -433,7 +432,7 @@ static const __initconst struct idt_data def_idts[] = {
|
||||
INTG(X86_TRAP_DF, double_fault),
|
||||
```
|
||||
|
||||
where `nmi` and `double_fault` are entry points created at [arch/x86/kernel/entry_64.S](https://github.com/torvalds/linux/blob/master/arch/x86/entry/entry_64.S):
|
||||
where `nmi` and `double_fault` are entry points created at [arch/x86/kernel/entry_64.S](https://github.com/torvalds/linux/blob/master/arch/x86/entry/entry_64.S):
|
||||
|
||||
```assembly
|
||||
idtentry double_fault do_double_fault has_error_code=1 paranoid=2 read_cr2=1
|
||||
|
||||
@ -6,7 +6,7 @@ Last part
|
||||
|
||||
This is the tenth part of the [chapter](https://0xax.gitbook.io/linux-insides/summary/interrupts) about interrupts and interrupt handling in the Linux kernel and in the previous [part](https://0xax.gitbook.io/linux-insides/summary/interrupts/linux-interrupts-9) we saw a little about deferred interrupts and related concepts like `softirq`, `tasklet` and `workqeue`. In this part we will continue to dive into this theme and now it's time to look at real hardware driver.
|
||||
|
||||
Let's consider serial driver of the [StrongARM** SA-110/21285 Evaluation Board](http://netwinder.osuosl.org/pub/netwinder/docs/intel/datashts/27813501.pdf) board for example and will look how this driver requests an [IRQ](https://en.wikipedia.org/wiki/Interrupt_request_%28PC_architecture%29) line,
|
||||
Let's consider serial driver of the [StrongARM** SA-110/21285 Evaluation Board](http://netwinder.osuosl.org/pub/netwinder/docs/intel/datashts/27813501.pdf) board for example and will look how this driver requests an [IRQ](https://en.wikipedia.org/wiki/Interrupt_request_%28PC_architecture%29) line,
|
||||
what happens when an interrupt is triggered and etc. The source code of this driver is placed in the [drivers/tty/serial/21285.c](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/drivers/tty/serial/21285.c) source code file. Ok, we have source code, let's start.
|
||||
|
||||
Initialization of a kernel module
|
||||
@ -111,7 +111,7 @@ if (ret == 0)
|
||||
return ret;
|
||||
```
|
||||
|
||||
That's all. Our driver is initialized. When an `uart` port will be opened with the call of the `uart_open` function from the [drivers/tty/serial/serial_core.c](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/drivers/tty/serial/serial_core.c), it will call the `uart_startup` function to start up the serial port. This function will call the `startup` function that is part of the `uart_ops` structure. Each `uart` driver has the definition of this structure, in our case it is:
|
||||
That's all. Our driver is initialized. When an `uart` port is opened with the call of the `uart_open` function from the [drivers/tty/serial/serial_core.c](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/drivers/tty/serial/serial_core.c), it will call the `uart_startup` function to start up the serial port. This function will call the `startup` function that is part of the `uart_ops` structure. Each `uart` driver has the definition of this structure, in our case it is:
|
||||
|
||||
```C
|
||||
static struct uart_ops serial21285_ops = {
|
||||
@ -243,7 +243,7 @@ if (!irq_settings_can_request(desc) || WARN_ON(irq_settings_is_per_cpu_devid(des
|
||||
return -EINVAL;
|
||||
```
|
||||
|
||||
and exit with the `-EINVAL`otherways. After this we check the given interrupt handler. If it was not passed to the `request_irq` function, we check the `thread_fn`. If both handlers are `NULL`, we return with the `-EINVAL`. If an interrupt handler was not passed to the `request_irq` function, but the `thread_fn` is not null, we set handler to the `irq_default_primary_handler`:
|
||||
and exit with the `-EINVAL` otherwise. After this we check the given interrupt handler. If it was not passed to the `request_irq` function, we check the `thread_fn`. If both handlers are `NULL`, we return with the `-EINVAL`. If an interrupt handler was not passed to the `request_irq` function, but the `thread_fn` is not null, we set handler to the `irq_default_primary_handler`:
|
||||
|
||||
```C
|
||||
if (!handler) {
|
||||
@ -296,12 +296,12 @@ if (new->thread_fn && !nested) {
|
||||
}
|
||||
```
|
||||
|
||||
And fill the rest of the given interrupt descriptor fields in the end. So, our `16` and `17` interrupt request lines are registered and the `serial21285_rx_chars` and `serial21285_tx_chars` functions will be invoked when an interrupt controller will get event related to these interrupts. Now let's look at what happens when an interrupt occurs.
|
||||
And fill the rest of the given interrupt descriptor fields in the end. So, our `16` and `17` interrupt request lines are registered and the `serial21285_rx_chars` and `serial21285_tx_chars` functions will be invoked when an interrupt controller will get event related to these interrupts. Now let's look at what happens when an interrupt occurs.
|
||||
|
||||
Prepare to handle an interrupt
|
||||
--------------------------------------------------------------------------------
|
||||
|
||||
In the previous paragraph we saw the requesting of the irq line for the given interrupt descriptor and registration of the `irqaction` structure for the given interrupt. We already know that when an interrupt event occurs, an interrupt controller notifies the processor about this event and processor tries to find appropriate interrupt gate for this interrupt. If you have read the eighth [part](https://0xax.gitbook.io/linux-insides/summary/interrupts/linux-interrupts-8) of this chapter, you may remember the `native_init_IRQ` function. This function makes initialization of the local [APIC](https://en.wikipedia.org/wiki/Advanced_Programmable_Interrupt_Controller). The following part of this function is the most interesting part for us right now:
|
||||
In the previous paragraph we saw the requesting of the irq line for the given interrupt descriptor and registration of the `irqaction` structure for the given interrupt. We already know that when an interrupt event occurs, an interrupt controller notifies the processor about this event and processor tries to find appropriate interrupt gate for this interrupt. If you have read the eighth [part](https://0xax.gitbook.io/linux-insides/summary/interrupts/linux-interrupts-8) of this chapter, you may remember the `native_init_IRQ` function. This function makes initialization of the local [APIC](https://en.wikipedia.org/wiki/Advanced_Programmable_Interrupt_Controller). The following part of this function is the most interesting part for us right now:
|
||||
|
||||
```C
|
||||
for_each_clear_bit_from(i, used_vectors, first_system_vector) {
|
||||
@ -398,7 +398,7 @@ static inline void generic_handle_irq_desc(unsigned int irq, struct irq_desc *de
|
||||
}
|
||||
```
|
||||
|
||||
But stop... What is it `handle_irq` and why do we call our interrupt handler from the interrupt descriptor when we know that `irqaction` points to the actual interrupt handler? Actually the `irq_desc->handle_irq` is a high-level API for the calling interrupt handler routine. It setups during initialization of the [device tree](https://en.wikipedia.org/wiki/Device_tree) and [APIC](https://en.wikipedia.org/wiki/Advanced_Programmable_Interrupt_Controller) initialization. The kernel selects correct function and call chain of the `irq->action(s)` there. In this way, the `serial21285_tx_chars` or the `serial21285_rx_chars` function will be executed after an interrupt will occur.
|
||||
But stop... What is it `handle_irq` and why do we call our interrupt handler from the interrupt descriptor when we know that `irqaction` points to the actual interrupt handler? Actually the `irq_desc->handle_irq` is a high-level API for the calling interrupt handler routine. It is setup during initialization of the [device tree](https://en.wikipedia.org/wiki/Device_tree) and [APIC](https://en.wikipedia.org/wiki/Advanced_Programmable_Interrupt_Controller) initialization. The kernel selects correct function and call chain of the `irq->action(s)` there. In this way, the `serial21285_tx_chars` or the `serial21285_rx_chars` function will be executed after an interrupt occurs.
|
||||
|
||||
In the end of the `do_IRQ` function we call the `irq_exit` function that will exit from the interrupt context, the `set_irq_regs` with the old userspace registers and return:
|
||||
|
||||
@ -413,7 +413,7 @@ We already know that when an `IRQ` finishes its work, deferred interrupts will b
|
||||
Exit from interrupt
|
||||
--------------------------------------------------------------------------------
|
||||
|
||||
Ok, the interrupt handler finished its execution and now we must return from the interrupt. When the work of the `do_IRQ` function will be finished, we will return back to the assembler code in the [arch/x86/entry/entry_64.S](https://github.com/torvalds/linux/blob/master/arch/x86/entry/entry_64.S) to the `ret_from_intr` label. First of all we disable interrupts with the `DISABLE_INTERRUPTS` macro that expands to the `cli` instruction and decreases value of the `irq_count` [per-cpu](https://0xax.gitbook.io/linux-insides/summary/concepts/linux-cpu-1) variable. Remember, this variable had value - `1`, when we were in interrupt context:
|
||||
Ok, the interrupt handler finished its execution and now we must return from the interrupt. When the work of the `do_IRQ` function is finished, we will return back to the assembler code in the [arch/x86/entry/entry_64.S](https://github.com/torvalds/linux/blob/master/arch/x86/entry/entry_64.S) to the `ret_from_intr` label. First of all we disable interrupts with the `DISABLE_INTERRUPTS` macro that expands to the `cli` instruction and decreases value of the `irq_count` [per-cpu](https://0xax.gitbook.io/linux-insides/summary/concepts/linux-cpu-1) variable. Remember, this variable had value - `1`, when we were in interrupt context:
|
||||
|
||||
```assembly
|
||||
DISABLE_INTERRUPTS(CLBR_NONE)
|
||||
@ -462,8 +462,8 @@ Links
|
||||
* [IRQ](https://en.wikipedia.org/wiki/Interrupt_request_%28PC_architecture%29)
|
||||
* [module](https://en.wikipedia.org/wiki/Loadable_kernel_module)
|
||||
* [initcall](http://kernelnewbies.org/Documents/InitcallMechanism)
|
||||
* [uart](https://en.wikipedia.org/wiki/Universal_asynchronous_receiver/transmitter)
|
||||
* [ISA](https://en.wikipedia.org/wiki/Industry_Standard_Architecture)
|
||||
* [uart](https://en.wikipedia.org/wiki/Universal_asynchronous_receiver/transmitter)
|
||||
* [ISA](https://en.wikipedia.org/wiki/Industry_Standard_Architecture)
|
||||
* [memory management](https://0xax.gitbook.io/linux-insides/summary/mm)
|
||||
* [i2c](https://en.wikipedia.org/wiki/I%C2%B2C)
|
||||
* [APIC](https://en.wikipedia.org/wiki/Advanced_Programmable_Interrupt_Controller)
|
||||
|
||||
@ -196,7 +196,7 @@ for (i = 0; i < NUM_EXCEPTION_VECTORS; i++)
|
||||
load_idt((const struct desc_ptr *)&idt_descr);
|
||||
```
|
||||
|
||||
AS you can see it has only one difference in the name of the array of the interrupts handlers entry points. Now it is `early_idt_handler_array`:
|
||||
As you can see it has only one difference in the name of the array of the interrupts handlers entry points. Now it is `early_idt_handler_array`:
|
||||
|
||||
```C
|
||||
extern const char early_idt_handler_array[NUM_EXCEPTION_VECTORS][EARLY_IDT_HANDLER_SIZE];
|
||||
@ -209,7 +209,7 @@ where `NUM_EXCEPTION_VECTORS` and `EARLY_IDT_HANDLER_SIZE` are defined as:
|
||||
#define EARLY_IDT_HANDLER_SIZE 9
|
||||
```
|
||||
|
||||
So, the `early_idt_handler_array` is an array of the interrupts handlers entry points and contains one entry point on every nine bytes. You can remember that previous `early_idt_handlers` was defined in the [arch/x86/kernel/head_64.S](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/arch/x86/kernel/head_64.S). The `early_idt_handler_array` is defined in the same source code file too:
|
||||
So, the `early_idt_handler_array` is an array of the interrupts handlers entry points and contains one entry point on every nine bytes. You can remember that previous `early_idt_handlers` was defined in the [arch/x86/kernel/head_64.S](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/arch/x86/kernel/head_64.S). The `early_idt_handler_array` is defined in the same source code file too:
|
||||
|
||||
```assembly
|
||||
ENTRY(early_idt_handler_array)
|
||||
@ -357,7 +357,7 @@ $ sudo cat /proc/lockdep
|
||||
redundant softirq offs: 0
|
||||
```
|
||||
|
||||
Ok, now we know a little about tracing, but more info will be in the separate part about `lockdep` and `tracing`. You can see that the both `local_disable_irq` macros have the same part - `raw_local_irq_disable`. This macro defined in the [arch/x86/include/asm/irqflags.h](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/arch/x86/include/asm/irqflags.h) and expands to the call of the:
|
||||
Ok, now we know a little about tracing, but more info will be in the separate part about `lockdep` and `tracing`. You can see that the both `local_irq_disable` macros have the same part - `raw_local_irq_disable`. This macro defined in the [arch/x86/include/asm/irqflags.h](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/arch/x86/include/asm/irqflags.h) and expands to the call of the:
|
||||
|
||||
```C
|
||||
static inline void native_irq_disable(void)
|
||||
@ -417,7 +417,7 @@ Here we can see calls of three different functions:
|
||||
* `set_system_intr_gate_ist`
|
||||
* `set_intr_gate`
|
||||
|
||||
All of these functions defined in the [arch/x86/include/asm/desc.h](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/arch/x86/include/asm/desc.h) and do the similar thing but not the same. The first `set_intr_gate_ist` function inserts new an interrupt gate in the `IDT`. Let's look on its implementation:
|
||||
All of these functions defined in the [arch/x86/include/asm/desc.h](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/arch/x86/include/asm/desc.h) and do the similar thing but not the same. The first `set_intr_gate_ist` function inserts a new interrupt gate in the `IDT`. Let's look on its implementation:
|
||||
|
||||
```C
|
||||
static inline void set_intr_gate_ist(int n, void *addr, unsigned ist)
|
||||
@ -494,7 +494,7 @@ static inline void set_system_intr_gate_ist(int n, void *addr, unsigned ist)
|
||||
}
|
||||
```
|
||||
|
||||
Do you see it? Look on the fourth parameter of the `_set_gate`. It is `0x3`. In the `set_intr_gate` it was `0x0`. We know that this parameter represent `DPL` or privilege level. We also know that `0` is the highest privilege level and `3` is the lowest.Now we know how `set_system_intr_gate_ist`, `set_intr_gate_ist`, `set_intr_gate` are work and we can return to the `early_trap_init` function. Let's look on it again:
|
||||
Do you see it? Look on the fourth parameter of the `_set_gate`. It is `0x3`. In the `set_intr_gate` it was `0x0`. We know that this parameter represent `DPL` or privilege level. We also know that `0` is the highest privilege level and `3` is the lowest. Now we know how `set_system_intr_gate_ist`, `set_intr_gate_ist`, `set_intr_gate` work and we can return to the `early_trap_init` function. Let's look on it again:
|
||||
|
||||
```C
|
||||
set_intr_gate_ist(X86_TRAP_DB, &debug, DEBUG_STACK);
|
||||
|
||||
@ -4,9 +4,9 @@ Interrupts and Interrupt Handling. Part 3.
|
||||
Exception Handling
|
||||
--------------------------------------------------------------------------------
|
||||
|
||||
This is the third part of the [chapter](https://0xax.gitbook.io/linux-insides/summary/interrupts) about an interrupts and an exceptions handling in the Linux kernel and in the previous [part](https://0xax.gitbook.io/linux-insides/summary/interrupts) we stopped at the `setup_arch` function from the [arch/x86/kernel/setup.c](https://github.com/torvalds/linux/blame/master/arch/x86/kernel/setup.c) source code file.
|
||||
This is the third part of the [chapter](https://0xax.gitbook.io/linux-insides/summary/interrupts) about interrupts and an exceptions handling in the Linux kernel and in the previous [part](https://0xax.gitbook.io/linux-insides/summary/interrupts) we stopped at the `setup_arch` function from the [arch/x86/kernel/setup.c](https://github.com/torvalds/linux/blame/master/arch/x86/kernel/setup.c) source code file.
|
||||
|
||||
We already know that this function executes initialization of architecture-specific stuff. In our case the `setup_arch` function does [x86_64](https://en.wikipedia.org/wiki/X86-64) architecture related initializations. The `setup_arch` is big function, and in the previous part we stopped on the setting of the two exceptions handlers for the two following exceptions:
|
||||
We already know that this function executes initialization of architecture-specific stuff. In our case the `setup_arch` function does [x86_64](https://en.wikipedia.org/wiki/X86-64) architecture related initializations. The `setup_arch` is big function, and in the previous part we stopped on the setting of the two exception handlers for the two following exceptions:
|
||||
|
||||
* `#DB` - debug exception, transfers control from the interrupted process to the debug handler;
|
||||
* `#BP` - breakpoint exception, caused by the `int 3` instruction.
|
||||
@ -24,18 +24,18 @@ void __init early_trap_init(void)
|
||||
}
|
||||
```
|
||||
|
||||
from the [arch/x86/kernel/traps.c](https://github.com/torvalds/linux/tree/master/arch/x86/kernel/traps.c). We already saw implementation of the `set_intr_gate_ist` and `set_system_intr_gate_ist` functions in the previous part and now we will look on the implementation of these two exceptions handlers.
|
||||
from the [arch/x86/kernel/traps.c](https://github.com/torvalds/linux/tree/master/arch/x86/kernel/traps.c). We already saw implementation of the `set_intr_gate_ist` and `set_system_intr_gate_ist` functions in the previous part and now we will look on the implementation of these two exception handlers.
|
||||
|
||||
Debug and Breakpoint exceptions
|
||||
--------------------------------------------------------------------------------
|
||||
|
||||
Ok, we setup exception handlers in the `early_trap_init` function for the `#DB` and `#BP` exceptions and now time is to consider their implementations. But before we will do this, first of all let's look on details of these exceptions.
|
||||
Ok, we setup exception handlers in the `early_trap_init` function for the `#DB` and `#BP` exceptions and now is time to consider their implementations. But before we will do this, first of all let's look on details of these exceptions.
|
||||
|
||||
The first exceptions - `#DB` or `debug` exception occurs when a debug event occurs. For example - attempt to change the contents of a [debug register](http://en.wikipedia.org/wiki/X86_debug_register). Debug registers are special registers that were presented in `x86` processors starting from the [Intel 80386](http://en.wikipedia.org/wiki/Intel_80386) processor and as you can understand from name of this CPU extension, main purpose of these registers is debugging.
|
||||
|
||||
These registers allow to set breakpoints on the code and read or write data to trace it. Debug registers may be accessed only in the privileged mode and an attempt to read or write the debug registers when executing at any other privilege level causes a [general protection fault](https://en.wikipedia.org/wiki/General_protection_fault) exception. That's why we have used `set_intr_gate_ist` for the `#DB` exception, but not the `set_system_intr_gate_ist`.
|
||||
|
||||
The verctor number of the `#DB` exceptions is `1` (we pass it as `X86_TRAP_DB`) and as we may read in specification, this exception has no error code:
|
||||
The vector number of the `#DB` exceptions is `1` (we pass it as `X86_TRAP_DB`) and as we may read in specification, this exception has no error code:
|
||||
|
||||
```
|
||||
+-----------------------------------------------------+
|
||||
@ -65,6 +65,7 @@ If we will compile and run this program, we will see following output:
|
||||
|
||||
```
|
||||
$ gcc breakpoint.c -o breakpoint
|
||||
$ ./breakpoint
|
||||
i equal to: 0
|
||||
Trace/breakpoint trap
|
||||
```
|
||||
@ -77,7 +78,7 @@ $ gdb breakpoint
|
||||
...
|
||||
...
|
||||
(gdb) run
|
||||
Starting program: /home/alex/breakpoints
|
||||
Starting program: /home/alex/breakpoints
|
||||
i equal to: 0
|
||||
|
||||
Program received signal SIGTRAP, Trace/breakpoint trap.
|
||||
@ -112,7 +113,7 @@ As you may note before, the `set_intr_gate_ist` and `set_system_intr_gate_ist` f
|
||||
* `debug`;
|
||||
* `int3`.
|
||||
|
||||
You will not find these functions in the C code. all of that could be found in the kernel's `*.c/*.h` files only definition of these functions which are located in the [arch/x86/include/asm/traps.h](https://github.com/torvalds/linux/tree/master/arch/x86/include/asm/traps.h) kernel header file:
|
||||
You will not find these functions in the C code. All of that could be found in the kernel's `*.c/*.h` files only definition of these functions which are located in the [arch/x86/include/asm/traps.h](https://github.com/torvalds/linux/tree/master/arch/x86/include/asm/traps.h) kernel header file:
|
||||
|
||||
```C
|
||||
asmlinkage void debug(void);
|
||||
@ -138,7 +139,7 @@ and
|
||||
idtentry int3 do_int3 has_error_code=0 paranoid=1 shift_ist=DEBUG_STACK
|
||||
```
|
||||
|
||||
Each exception handler may be consists from two parts. The first part is generic part and it is the same for all exception handlers. An exception handler should to save [general purpose registers](https://en.wikipedia.org/wiki/Processor_register) on the stack, switch to kernel stack if an exception came from userspace and transfer control to the second part of an exception handler. The second part of an exception handler does certain work depends on certain exception. For example page fault exception handler should find virtual page for given address, invalid opcode exception handler should send `SIGILL` [signal](https://en.wikipedia.org/wiki/Unix_signal) and etc.
|
||||
Each exception handler may consists of two parts. The first part is generic part and it is the same for all exception handlers. An exception handler should to save [general purpose registers](https://en.wikipedia.org/wiki/Processor_register) on the stack, switch to kernel stack if an exception came from userspace and transfer control to the second part of an exception handler. The second part of an exception handler does certain work depends on certain exception. For example page fault exception handler should find virtual page for given address, invalid opcode exception handler should send `SIGILL` [signal](https://en.wikipedia.org/wiki/Unix_signal) and etc.
|
||||
|
||||
As we just saw, an exception handler starts from definition of the `idtentry` macro from the [arch/x86/entry/entry_64.S](https://github.com/torvalds/linux/blob/master/arch/x86/entry/entry_64.S) assembly source code file, so let's look at implementation of this macro. As we may see, the `idtentry` macro takes five arguments:
|
||||
|
||||
@ -193,7 +194,7 @@ If we will look at these definitions, we may know that compiler will generate tw
|
||||
|
||||
But it is not only fake error-code. Moreover the `-1` also represents invalid system call number, so that the system call restart logic will not be triggered.
|
||||
|
||||
The last two parameters of the `idtentry` macro `shift_ist` and `paranoid` allow to know do an exception handler runned at stack from `Interrupt Stack Table` or not. You already may know that each kernel thread in the system has own stack. In addition to these stacks, there are some specialized stacks associated with each processor in the system. One of these stacks is - exception stack. The [x86_64](https://en.wikipedia.org/wiki/X86-64) architecture provides special feature which is called - `Interrupt Stack Table`. This feature allows to switch to a new stack for designated events such as an atomic exceptions like `double fault` and etc. So the `shift_ist` parameter allows us to know do we need to switch on `IST` stack for an exception handler or not.
|
||||
The last two parameters of the `idtentry` macro `shift_ist` and `paranoid` allow to know do an exception handler runned at stack from `Interrupt Stack Table` or not. You already may know that each kernel thread in the system has its own stack. In addition to these stacks, there are some specialized stacks associated with each processor in the system. One of these stacks is - exception stack. The [x86_64](https://en.wikipedia.org/wiki/X86-64) architecture provides special feature which is called - `Interrupt Stack Table`. This feature allows to switch to a new stack for designated events such as an atomic exceptions like `double fault`, etc. So the `shift_ist` parameter allows us to know do we need to switch on `IST` stack for an exception handler or not.
|
||||
|
||||
The second parameter - `paranoid` defines the method which helps us to know did we come from userspace or not to an exception handler. The easiest way to determine this is to via `CPL` or `Current Privilege Level` in `CS` segment register. If it is equal to `3`, we came from userspace, if zero we came from kernel space:
|
||||
|
||||
@ -213,7 +214,7 @@ But unfortunately this method does not give a 100% guarantee. As described in th
|
||||
> stack but before we executed SWAPGS, then the only safe way to check
|
||||
> for GS is the slower method: the RDMSR.
|
||||
|
||||
In other words for example `NMI` could happen inside the critical section of a [swapgs](http://www.felixcloutier.com/x86/SWAPGS.html) instruction. In this way we should check value of the `MSR_GS_BASE` [model specific register](https://en.wikipedia.org/wiki/Model-specific_register) which stores pointer to the start of per-cpu area. So to check did we come from userspace or not, we should to check value of the `MSR_GS_BASE` model specific register and if it is negative we came from kernel space, in other way we came from userspace:
|
||||
In other words for example `NMI` could happen inside the critical section of a [swapgs](http://www.felixcloutier.com/x86/SWAPGS.html) instruction. In this way we should check value of the `MSR_GS_BASE` [model specific register](https://en.wikipedia.org/wiki/Model-specific_register) which stores pointer to the start of per-cpu area. So to check if we did come from userspace or not, we should to check value of the `MSR_GS_BASE` model specific register and if it is negative we came from kernel space, in other way we came from userspace:
|
||||
|
||||
```assembly
|
||||
movl $MSR_GS_BASE,%ecx
|
||||
@ -224,7 +225,7 @@ js 1f
|
||||
|
||||
In first two lines of code we read value of the `MSR_GS_BASE` model specific register into `edx:eax` pair. We can't set negative value to the `gs` from userspace. But from other side we know that direct mapping of the physical memory starts from the `0xffff880000000000` virtual address. In this way, `MSR_GS_BASE` will contain an address from `0xffff880000000000` to `0xffffc7ffffffffff`. After the `rdmsr` instruction will be executed, the smallest possible value in the `%edx` register will be - `0xffff8800` which is `-30720` in unsigned 4 bytes. That's why kernel space `gs` which points to start of `per-cpu` area will contain negative value.
|
||||
|
||||
After we pushed fake error code on the stack, we should allocate space for general purpose registers with:
|
||||
After we push fake error code on the stack, we should allocate space for general purpose registers with:
|
||||
|
||||
```assembly
|
||||
ALLOC_PT_GPREGS_ON_STACK
|
||||
@ -370,7 +371,7 @@ asmlinkage __visible notrace struct pt_regs *sync_regs(struct pt_regs *eregs)
|
||||
}
|
||||
```
|
||||
|
||||
This function takes the result of the `task_ptr_regs` macro which is defined in the [arch/x86/include/asm/processor.h](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/arch/x86/include/asm/processor.h) header file, stores it in the stack pointer and return it. The `task_ptr_regs` macro expands to the address of `thread.sp0` which represents pointer to the normal kernel stack:
|
||||
This function takes the result of the `task_ptr_regs` macro which is defined in the [arch/x86/include/asm/processor.h](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/arch/x86/include/asm/processor.h) header file, stores it in the stack pointer and returns it. The `task_ptr_regs` macro expands to the address of `thread.sp0` which represents pointer to the normal kernel stack:
|
||||
|
||||
```C
|
||||
#define task_pt_regs(tsk) ((struct pt_regs *)(tsk)->thread.sp0 - 1)
|
||||
@ -403,7 +404,7 @@ as it will be passed as first parameter of secondary exception handler.
|
||||
.endif
|
||||
```
|
||||
|
||||
Additionally you may see that we zeroed the `%esi` register above in a case if an exception does not provide error code.
|
||||
Additionally you may see that we zeroed the `%esi` register above in a case if an exception does not provide error code.
|
||||
|
||||
In the end we just call secondary exception handler:
|
||||
|
||||
@ -423,7 +424,7 @@ will be for `debug` exception and:
|
||||
dotraplinkage void notrace do_int3(struct pt_regs *regs, long error_code);
|
||||
```
|
||||
|
||||
will be for `int 3` exception. In this part we will not see implementations of secondary handlers, because of they are very specific, but will see some of them in one of next parts.
|
||||
will be for `int 3` exception. In this part we will not see implementations of secondary handlers, because they are very specific, but will see some of them in one of next parts.
|
||||
|
||||
We just considered first case when an exception occurred in userspace. Let's consider last two.
|
||||
|
||||
@ -461,7 +462,7 @@ movq %rsp, %rdi
|
||||
.endif
|
||||
```
|
||||
|
||||
The last step before a secondary handler of an exception will be called is cleanup of new `IST` stack fram:
|
||||
The last step before a secondary handler of an exception will be called is cleanup of new `IST` stack frame:
|
||||
|
||||
```assembly
|
||||
.if \shift_ist != -1
|
||||
|
||||
@ -99,7 +99,7 @@ do_page_fault(struct pt_regs *regs, unsigned long error_code)
|
||||
}
|
||||
```
|
||||
|
||||
This register contains a linear address which caused `page fault`. In the next step we make a call of the `exception_enter` function from the [include/linux/context_tracking.h](https://github.com/torvalds/linux/blob/master/include/linux/context_tracking.h). The `exception_enter` and `exception_exit` are functions from context tracking subsystem in the Linux kernel used by the [RCU](https://en.wikipedia.org/wiki/Read-copy-update) to remove its dependency on the timer tick while a processor runs in userspace. Almost in the every exception handler we will see similar code:
|
||||
This register contains a linear address which caused `page fault`. In the next step we make a call of the `exception_enter` function from the [include/linux/context_tracking.h](https://github.com/torvalds/linux/blob/master/include/linux/context_tracking.h). The `exception_enter` and `exception_exit` are functions from context tracking subsystem in the Linux kernel used by the [RCU](https://en.wikipedia.org/wiki/Read-copy-update) to remove its dependency on the timer tick while a processor runs in userspace. Almost in every exception handler we will see similar code:
|
||||
|
||||
```C
|
||||
enum ctx_state prev_state;
|
||||
@ -182,7 +182,7 @@ or `0x00007ffffffff000`. Pay attention on `unlikely` macro. There are two macros
|
||||
#define unlikely(x) __builtin_expect(!!(x), 0)
|
||||
```
|
||||
|
||||
You can [often](http://lxr.free-electrons.com/ident?i=unlikely) find these macros in the code of the Linux kernel. Main purpose of these macros is optimization. Sometimes this situation is that we need to check the condition of the code and we know that it will rarely be `true` or `false`. With these macros we can tell to the compiler about this. For example
|
||||
You can [often](http://lxr.free-electrons.com/ident?i=unlikely) find these macros in the code of the Linux kernel. Main purpose of these macros is optimization. Sometimes this situation is that we need to check the condition of the code and we know that it will rarely be `true` or `false`. With these macros we can tell to the compiler about this. For example
|
||||
|
||||
```C
|
||||
static int proc_root_readdir(struct file *file, struct dir_context *ctx)
|
||||
@ -447,7 +447,7 @@ Links
|
||||
* [prefetchw](http://www.felixcloutier.com/x86/PREFETCHW.html)
|
||||
* [3DNow](https://en.wikipedia.org/?title=3DNow!)
|
||||
* [CPU caches](https://en.wikipedia.org/wiki/CPU_cache)
|
||||
* [VFS](https://en.wikipedia.org/wiki/Virtual_file_system)
|
||||
* [VFS](https://en.wikipedia.org/wiki/Virtual_file_system)
|
||||
* [Linux kernel memory management](https://0xax.gitbook.io/linux-insides/summary/mm)
|
||||
* [Fix-Mapped Addresses and ioremap](https://0xax.gitbook.io/linux-insides/summary/mm/linux-mm-2)
|
||||
* [Extended Industry Standard Architecture](https://en.wikipedia.org/wiki/Extended_Industry_Standard_Architecture)
|
||||
|
||||
@ -7,21 +7,21 @@ Implementation of exception handlers
|
||||
This is the fifth part about an interrupts and exceptions handling in the Linux kernel and in the previous [part](https://0xax.gitbook.io/linux-insides/summary/interrupts/linux-interrupts-4) we stopped on the setting of interrupt gates to the [Interrupt descriptor Table](https://en.wikipedia.org/wiki/Interrupt_descriptor_table). We did it in the `trap_init` function from the [arch/x86/kernel/traps.c](https://github.com/torvalds/linux/tree/master/arch/x86/kernel/traps.c) source code file. We saw only setting of these interrupt gates in the previous part and in the current part we will see implementation of the exception handlers for these gates. The preparation before an exception handler will be executed is in the [arch/x86/entry/entry_64.S](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/arch/x86/entry/entry_64.S) assembly file and occurs in the [idtentry](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/arch/x86/entry/entry_64.S#L820) macro that defines exceptions entry points:
|
||||
|
||||
```assembly
|
||||
idtentry divide_error do_divide_error has_error_code=0
|
||||
idtentry overflow do_overflow has_error_code=0
|
||||
idtentry invalid_op do_invalid_op has_error_code=0
|
||||
idtentry bounds do_bounds has_error_code=0
|
||||
idtentry device_not_available do_device_not_available has_error_code=0
|
||||
idtentry coprocessor_segment_overrun do_coprocessor_segment_overrun has_error_code=0
|
||||
idtentry invalid_TSS do_invalid_TSS has_error_code=1
|
||||
idtentry segment_not_present do_segment_not_present has_error_code=1
|
||||
idtentry spurious_interrupt_bug do_spurious_interrupt_bug has_error_code=0
|
||||
idtentry coprocessor_error do_coprocessor_error has_error_code=0
|
||||
idtentry alignment_check do_alignment_check has_error_code=1
|
||||
idtentry simd_coprocessor_error do_simd_coprocessor_error has_error_code=0
|
||||
idtentry divide_error do_divide_error has_error_code=0
|
||||
idtentry overflow do_overflow has_error_code=0
|
||||
idtentry invalid_op do_invalid_op has_error_code=0
|
||||
idtentry bounds do_bounds has_error_code=0
|
||||
idtentry device_not_available do_device_not_available has_error_code=0
|
||||
idtentry coprocessor_segment_overrun do_coprocessor_segment_overrun has_error_code=0
|
||||
idtentry invalid_TSS do_invalid_TSS has_error_code=1
|
||||
idtentry segment_not_present do_segment_not_present has_error_code=1
|
||||
idtentry spurious_interrupt_bug do_spurious_interrupt_bug has_error_code=0
|
||||
idtentry coprocessor_error do_coprocessor_error has_error_code=0
|
||||
idtentry alignment_check do_alignment_check has_error_code=1
|
||||
idtentry simd_coprocessor_error do_simd_coprocessor_error has_error_code=0
|
||||
```
|
||||
|
||||
The `idtentry` macro does following preparation before an actual exception handler (`do_divide_error` for the `divide_error`, `do_overflow` for the `overflow` and etc.) will get control. In another words the `idtentry` macro allocates place for the registers ([pt_regs](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/arch/x86/include/uapi/asm/ptrace.h#L43) structure) on the stack, pushes dummy error code for the stack consistency if an interrupt/exception has no error code, checks the segment selector in the `cs` segment register and switches depends on the previous state(userspace or kernelspace). After all of these preparations it makes a call of an actual interrupt/exception handler:
|
||||
The `idtentry` macro does following preparation before an actual exception handler (`do_divide_error` for the `divide_error`, `do_overflow` for the `overflow`, etc.) will get control. In another words the `idtentry` macro allocates place for the registers ([pt_regs](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/arch/x86/include/uapi/asm/ptrace.h#L43) structure) on the stack, pushes dummy error code for the stack consistency if an interrupt/exception has no error code, checks the segment selector in the `cs` segment register and switches depends on the previous state (userspace or kernelspace). After all of these preparations it makes a call to an actual interrupt/exception handler:
|
||||
|
||||
```assembly
|
||||
.macro idtentry sym do_sym has_error_code:req paranoid=0 shift_ist=-1
|
||||
@ -84,7 +84,7 @@ DO_ERROR(X86_TRAP_TS, SIGSEGV, "invalid TSS", invalid_TSS)
|
||||
DO_ERROR(X86_TRAP_NP, SIGBUS, "segment not present", segment_not_present)
|
||||
DO_ERROR(X86_TRAP_SS, SIGBUS, "stack segment", stack_segment)
|
||||
DO_ERROR(X86_TRAP_AC, SIGBUS, "alignment check", alignment_check)
|
||||
```
|
||||
```
|
||||
|
||||
As we can see the `DO_ERROR` macro takes 4 parameters:
|
||||
|
||||
@ -112,7 +112,7 @@ dotraplinkage void do_divide_error(struct pt_regs *regs, long error_code) \
|
||||
}
|
||||
```
|
||||
|
||||
We can see that all functions which are generated by the `DO_ERROR` macro just make a call of the `do_error_trap` function from the [arch/x86/kernel/traps.c](https://github.com/torvalds/linux/tree/master/arch/x86/kernel/traps.c). Let's look on implementation of the `do_error_trap` function.
|
||||
We can see that all functions which are generated by the `DO_ERROR` macro just make a call to the `do_error_trap` function from the [arch/x86/kernel/traps.c](https://github.com/torvalds/linux/tree/master/arch/x86/kernel/traps.c). Let's look on implementation of the `do_error_trap` function.
|
||||
|
||||
Trap handlers
|
||||
--------------------------------------------------------------------------------
|
||||
@ -150,7 +150,7 @@ enum ctx_state {
|
||||
} state;
|
||||
```
|
||||
|
||||
The second function is `exception_exit` defined in the same [include/linux/context_tracking.h](https://github.com/torvalds/linux/tree/master/include/linux/context_tracking.h) file and checks that context tracking is enabled and call the `contert_tracking_enter` function if the previous context was `user`:
|
||||
The second function is `exception_exit` defined in the same [include/linux/context_tracking.h](https://github.com/torvalds/linux/tree/master/include/linux/context_tracking.h) file and checks that context tracking is enabled and call the `context_tracking_enter` function if the previous context was `user`:
|
||||
|
||||
```C
|
||||
static inline void exception_exit(enum ctx_state prev_ctx)
|
||||
@ -173,7 +173,7 @@ if (notify_die(DIE_TRAP, str, regs, error_code, trapnr, signr) !=
|
||||
}
|
||||
```
|
||||
|
||||
First of all it calls the `notify_die` function which defined in the [kernel/notifier.c](https://github.com/torvalds/linux/tree/master/kernel/notifier.c). To get notified for [kernel panic](https://en.wikipedia.org/wiki/Kernel_panic), [kernel oops](https://en.wikipedia.org/wiki/Linux_kernel_oops), [Non-Maskable Interrupt](https://en.wikipedia.org/wiki/Non-maskable_interrupt) or other events the caller needs to insert itself in the `notify_die` chain and the `notify_die` function does it. The Linux kernel has special mechanism that allows kernel to ask when something happens and this mechanism called `notifiers` or `notifier chains`. This mechanism used for example for the `USB` hotplug events (look on the [drivers/usb/core/notify.c](https://github.com/torvalds/linux/tree/master/drivers/usb/core/notify.c)), for the memory [hotplug](https://en.wikipedia.org/wiki/Hot_swapping) (look on the [include/linux/memory.h](https://github.com/torvalds/linux/tree/master/include/linux/memory.h), the `hotplug_memory_notifier` macro and etc...), system reboots and etc. A notifier chain is thus a simple, singly-linked list. When a Linux kernel subsystem wants to be notified of specific events, it fills out a special `notifier_block` structure and passes it to the `notifier_chain_register` function. An event can be sent with the call of the `notifier_call_chain` function. First of all the `notify_die` function fills `die_args` structure with the trap number, trap string, registers and other values:
|
||||
First of all it calls the `notify_die` function which defined in the [kernel/notifier.c](https://github.com/torvalds/linux/tree/master/kernel/notifier.c). To get notified for [kernel panic](https://en.wikipedia.org/wiki/Kernel_panic), [kernel oops](https://en.wikipedia.org/wiki/Linux_kernel_oops), [Non-Maskable Interrupt](https://en.wikipedia.org/wiki/Non-maskable_interrupt) or other events the caller needs to insert itself in the `notify_die` chain and the `notify_die` function does it. The Linux kernel has special mechanism that allows kernel to ask when something happens and this mechanism called `notifiers` or `notifier chains`. This mechanism used for example for the `USB` hotplug events (look on the [drivers/usb/core/notify.c](https://github.com/torvalds/linux/tree/master/drivers/usb/core/notify.c)), for the memory [hotplug](https://en.wikipedia.org/wiki/Hot_swapping) (look on the [include/linux/memory.h](https://github.com/torvalds/linux/tree/master/include/linux/memory.h), the `hotplug_memory_notifier` macro, etc...), system reboots, etc. A notifier chain is thus a simple, singly-linked list. When a Linux kernel subsystem wants to be notified of specific events, it fills out a special `notifier_block` structure and passes it to the `notifier_chain_register` function. An event can be sent with the call of the `notifier_call_chain` function. First of all the `notify_die` function fills `die_args` structure with the trap number, trap string, registers and other values:
|
||||
|
||||
```C
|
||||
struct die_args args = {
|
||||
@ -247,7 +247,7 @@ if (!fixup_exception(regs)) {
|
||||
}
|
||||
```
|
||||
|
||||
The `die` function defined in the [arch/x86/kernel/dumpstack.c](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/arch/x86/kernel/dumpstack.c) source code file, prints useful information about stack, registers, kernel modules and caused kernel [oops](https://en.wikipedia.org/wiki/Linux_kernel_oops). If we came from the userspace the `do_trap_no_signal` function will return `-1` and the execution of the `do_trap` function will continue. If we passed through the `do_trap_no_signal` function and did not exit from the `do_trap` after this, it means that previous context was - `user`. Most exceptions caused by the processor are interpreted by Linux as error conditions, for example division by zero, invalid opcode and etc. When an exception occurs the Linux kernel sends a [signal](https://en.wikipedia.org/wiki/Unix_signal) to the interrupted process that caused the exception to notify it of an incorrect condition. So, in the `do_trap` function we need to send a signal with the given number (`SIGFPE` for the divide error, `SIGILL` for a illegal instruction and etc...). First of all we save error code and vector number in the current interrupts process with the filling `thread.error_code` and `thread_trap_nr`:
|
||||
The `die` function defined in the [arch/x86/kernel/dumpstack.c](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/arch/x86/kernel/dumpstack.c) source code file, prints useful information about stack, registers, kernel modules and caused kernel [oops](https://en.wikipedia.org/wiki/Linux_kernel_oops). If we came from the userspace the `do_trap_no_signal` function will return `-1` and the execution of the `do_trap` function will continue. If we passed through the `do_trap_no_signal` function and did not exit from the `do_trap` after this, it means that previous context was - `user`. Most exceptions caused by the processor are interpreted by Linux as error conditions, for example division by zero, invalid opcode, etc. When an exception occurs the Linux kernel sends a [signal](https://en.wikipedia.org/wiki/Unix_signal) to the interrupted process that caused the exception to notify it of an incorrect condition. So, in the `do_trap` function we need to send a signal with the given number (`SIGFPE` for the divide error, `SIGILL` for a illegal instruction, etc.). First of all we save error code and vector number in the current interrupts process with the filling `thread.error_code` and `thread_trap_nr`:
|
||||
|
||||
```C
|
||||
tsk->thread.error_code = error_code;
|
||||
@ -275,7 +275,7 @@ And send a given signal to interrupted process:
|
||||
force_sig_info(signr, info ?: SEND_SIG_PRIV, tsk);
|
||||
```
|
||||
|
||||
This is the end of the `do_trap`. We just saw generic implementation for eight different exceptions which are defined with the `DO_ERROR` macro. Now let's look on another exception handlers.
|
||||
This is the end of the `do_trap`. We just saw generic implementation for eight different exceptions which are defined with the `DO_ERROR` macro. Now let's look at other exception handlers.
|
||||
|
||||
Double fault
|
||||
--------------------------------------------------------------------------------
|
||||
|
||||
@ -4,7 +4,7 @@ Interrupts and Interrupt Handling. Part 6.
|
||||
Non-maskable interrupt handler
|
||||
--------------------------------------------------------------------------------
|
||||
|
||||
It is sixth part of the [Interrupts and Interrupt Handling in the Linux kernel](https://0xax.gitbook.io/linux-insides/summary/interrupts) chapter and in the previous [part](https://0xax.gitbook.io/linux-insides/summary/interrupts/linux-interrupts-5) we saw implementation of some exception handlers for the [General Protection Fault](https://en.wikipedia.org/wiki/General_protection_fault) exception, divide exception, invalid [opcode](https://en.wikipedia.org/wiki/Opcode) exceptions and etc. As I wrote in the previous part we will see implementations of the rest exceptions in this part. We will see implementation of the following handlers:
|
||||
It is sixth part of the [Interrupts and Interrupt Handling in the Linux kernel](https://0xax.gitbook.io/linux-insides/summary/interrupts) chapter and in the previous [part](https://0xax.gitbook.io/linux-insides/summary/interrupts/linux-interrupts-5) we saw implementation of some exception handlers for the [General Protection Fault](https://en.wikipedia.org/wiki/General_protection_fault) exception, divide exception, invalid [opcode](https://en.wikipedia.org/wiki/Opcode) exceptions, etc. As I wrote in the previous part we will see implementations of the rest exceptions in this part. We will see implementation of the following handlers:
|
||||
|
||||
* [Non-Maskable](https://en.wikipedia.org/wiki/Non-maskable_interrupt) interrupt;
|
||||
* [BOUND](http://pdos.csail.mit.edu/6.828/2005/readings/i386/BOUND.htm) Range Exceeded Exception;
|
||||
@ -169,7 +169,7 @@ pushq $-1
|
||||
ALLOC_PT_GPREGS_ON_STACK
|
||||
```
|
||||
|
||||
We already saw implementation of the `ALLOC_PT_GREGS_ON_STACK` macro in the third part of the interrupts [chapter](https://0xax.gitbook.io/linux-insides/summary/interrupts/linux-interrupts-3). This macro defined in the [arch/x86/entry/calling.h](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/arch/x86/entry/calling.h) and yet another allocates `120` bytes on stack for the general purpose registers, from the `rdi` to the `r15`:
|
||||
We already saw implementation of the `ALLOC_PT_GPREGS_ON_STACK` macro in the third part of the interrupts [chapter](https://0xax.gitbook.io/linux-insides/summary/interrupts/linux-interrupts-3). This macro defined in the [arch/x86/entry/calling.h](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/arch/x86/entry/calling.h) and yet another allocates `120` bytes on stack for the general purpose registers, from the `rdi` to the `r15`:
|
||||
|
||||
```assembly
|
||||
.macro ALLOC_PT_GPREGS_ON_STACK addskip=0
|
||||
@ -261,7 +261,7 @@ Now let's look on the `do_nmi` exception handler. This function defined in the [
|
||||
* error code.
|
||||
|
||||
as all exception handlers. The `do_nmi` starts from the call of the `nmi_nesting_preprocess` function and ends with the call of the `nmi_nesting_postprocess`. The `nmi_nesting_preprocess` function checks that we likely do not work with the debug stack and if we on the debug stack set the `update_debug_stack` [per-cpu](https://0xax.gitbook.io/linux-insides/summary/concepts/linux-cpu-1) variable to `1` and call the `debug_stack_set_zero` function from the [arch/x86/kernel/cpu/common.c](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/arch/x86/kernel/cpu/common.c). This function increases the `debug_stack_use_ctr` per-cpu variable and loads new `Interrupt Descriptor Table`:
|
||||
|
||||
|
||||
```C
|
||||
static inline void nmi_nesting_preprocess(struct pt_regs *regs)
|
||||
{
|
||||
@ -362,7 +362,7 @@ After all of this, there is still only one way when `MPX` is responsible for thi
|
||||
Coprocessor exception and SIMD exception
|
||||
--------------------------------------------------------------------------------
|
||||
|
||||
The next two exceptions are [x87 FPU](https://en.wikipedia.org/wiki/X87) Floating-Point Error exception or `#MF` and [SIMD](https://en.wikipedia.org/wiki/SIMD) Floating-Point Exception or `#XF`. The first exception occurs when the `x87 FPU` has detected floating point error. For example divide by zero, numeric overflow and etc. The second exception occurs when the processor has detected [SSE/SSE2/SSE3](https://en.wikipedia.org/wiki/SSE3) `SIMD` floating-point exception. It can be the same as for the `x87 FPU`. The handlers for these exceptions are `do_coprocessor_error` and `do_simd_coprocessor_error` are defined in the [arch/x86/kernel/traps.c](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/arch/x86/kernel/traps.c) and very similar on each other. They both make a call of the `math_error` function from the same source code file but pass different vector number. The `do_coprocessor_error` passes `X86_TRAP_MF` vector number to the `math_error`:
|
||||
The next two exceptions are [x87 FPU](https://en.wikipedia.org/wiki/X87) Floating-Point Error exception or `#MF` and [SIMD](https://en.wikipedia.org/wiki/SIMD) Floating-Point Exception or `#XF`. The first exception occurs when the `x87 FPU` has detected floating point error. For example divide by zero, numeric overflow, etc. The second exception occurs when the processor has detected [SSE/SSE2/SSE3](https://en.wikipedia.org/wiki/SSE3) `SIMD` floating-point exception. It can be the same as for the `x87 FPU`. The handlers for these exceptions are `do_coprocessor_error` and `do_simd_coprocessor_error` are defined in the [arch/x86/kernel/traps.c](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/arch/x86/kernel/traps.c) and very similar on each other. They both make a call of the `math_error` function from the same source code file but pass different vector number. The `do_coprocessor_error` passes `X86_TRAP_MF` vector number to the `math_error`:
|
||||
|
||||
```C
|
||||
dotraplinkage void do_coprocessor_error(struct pt_regs *regs, long error_code)
|
||||
@ -389,7 +389,7 @@ do_simd_coprocessor_error(struct pt_regs *regs, long error_code)
|
||||
}
|
||||
```
|
||||
|
||||
First of all the `math_error` function defines current interrupted task, address of its fpu, string which describes an exception, add it to the `notify_die` chain and return from the exception handler if it will return `NOTIFY_STOP`:
|
||||
First of all the `math_error` function defines current interrupted task, address of its FPU, string which describes an exception, add it to the `notify_die` chain and return from the exception handler if it will return `NOTIFY_STOP`:
|
||||
|
||||
```C
|
||||
struct task_struct *task = current;
|
||||
@ -434,7 +434,7 @@ After this we check the signal code and if it is non-zero we return:
|
||||
if (!info.si_code)
|
||||
return;
|
||||
```
|
||||
|
||||
|
||||
Or send the `SIGFPE` signal in the end:
|
||||
|
||||
```C
|
||||
@ -446,7 +446,7 @@ That's all.
|
||||
Conclusion
|
||||
--------------------------------------------------------------------------------
|
||||
|
||||
It is the end of the sixth part of the [Interrupts and Interrupt Handling](https://0xax.gitbook.io/linux-insides/summary/interrupts) chapter and we saw implementation of some exception handlers in this part, like `non-maskable` interrupt, [SIMD](https://en.wikipedia.org/wiki/SIMD) and [x87 FPU](https://en.wikipedia.org/wiki/X87) floating point exception. Finally we have finished with the `trap_init` function in this part and will go ahead in the next part. The next our point is the external interrupts and the `early_irq_init` function from the [init/main.c](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/init/main.c).
|
||||
It is the end of the sixth part of the [Interrupts and Interrupt Handling](https://0xax.gitbook.io/linux-insides/summary/interrupts) chapter and we saw implementation of some exception handlers in this part, like `non-maskable` interrupt, [SIMD](https://en.wikipedia.org/wiki/SIMD) and [x87 FPU](https://en.wikipedia.org/wiki/X87) floating point exception. Finally, we finished with the `trap_init` function in this part and will go ahead in the next part. The next our point is the external interrupts and the `early_irq_init` function from the [init/main.c](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/init/main.c).
|
||||
|
||||
If you have any questions or suggestions write me a comment or ping me at [twitter](https://twitter.com/0xAX).
|
||||
|
||||
@ -457,7 +457,7 @@ Links
|
||||
|
||||
* [General Protection Fault](https://en.wikipedia.org/wiki/General_protection_fault)
|
||||
* [opcode](https://en.wikipedia.org/wiki/Opcode)
|
||||
* [Non-Maskable](https://en.wikipedia.org/wiki/Non-maskable_interrupt)
|
||||
* [Non-Maskable](https://en.wikipedia.org/wiki/Non-maskable_interrupt)
|
||||
* [BOUND instruction](http://pdos.csail.mit.edu/6.828/2005/readings/i386/BOUND.htm)
|
||||
* [CPU socket](https://en.wikipedia.org/wiki/CPU_socket)
|
||||
* [Interrupt Descriptor Table](https://en.wikipedia.org/wiki/Interrupt_descriptor_table)
|
||||
@ -474,7 +474,7 @@ Links
|
||||
* [stack frame](https://en.wikipedia.org/wiki/Call_stack)
|
||||
* [Model Specific register](https://en.wikipedia.org/wiki/Model-specific_register)
|
||||
* [percpu](https://0xax.gitbook.io/linux-insides/summary/concepts/linux-cpu-1)
|
||||
* [RCU](https://en.wikipedia.org/wiki/Read-copy-update)
|
||||
* [RCU](https://en.wikipedia.org/wiki/Read-copy-update)
|
||||
* [MPX](https://en.wikipedia.org/wiki/Intel_MPX)
|
||||
* [x87 FPU](https://en.wikipedia.org/wiki/X87)
|
||||
* [Previous part](https://0xax.gitbook.io/linux-insides/summary/interrupts/linux-interrupts-5)
|
||||
|
||||
@ -4,9 +4,9 @@ Interrupts and Interrupt Handling. Part 7.
|
||||
Introduction to external interrupts
|
||||
--------------------------------------------------------------------------------
|
||||
|
||||
This is the seventh part of the Interrupts and Interrupt Handling in the Linux kernel [chapter](https://0xax.gitbook.io/linux-insides/summary/interrupts) and in the previous [part](https://0xax.gitbook.io/linux-insides/summary/interrupts/linux-interrupts-6) we have finished with the exceptions which are generated by the processor. In this part we will continue to dive to the interrupt handling and will start with the external hardware interrupt handling. As you can remember, in the previous part we have finished with the `trap_init` function from the [arch/x86/kernel/trap.c](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/arch/x86/kernel/traps.c) and the next step is the call of the `early_irq_init` function from the [init/main.c](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/init/main.c).
|
||||
This is the seventh part of the Interrupts and Interrupt Handling in the Linux kernel [chapter](https://0xax.gitbook.io/linux-insides/summary/interrupts) and in the previous [part](https://0xax.gitbook.io/linux-insides/summary/interrupts/linux-interrupts-6) we have finished with the exceptions which are generated by the processor. In this part we will continue to dive to the interrupt handling and will start with the external hardware interrupt handling. As you can remember, in the previous part we have finished with the `trap_init` function from the [arch/x86/kernel/trap.c](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/arch/x86/kernel/traps.c) and the next step is the call of the `early_irq_init` function from [init/main.c](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/init/main.c).
|
||||
|
||||
Interrupts are signal that are sent across [IRQ](https://en.wikipedia.org/wiki/Interrupt_request_%28PC_architecture%29) or `Interrupt Request Line` by a hardware or software. External hardware interrupts allow devices like keyboard, mouse and etc, to indicate that it needs attention of the processor. Once the processor receives the `Interrupt Request`, it will temporary stop execution of the running program and invoke special routine which depends on an interrupt. We already know that this routine is called interrupt handler (or how we will call it `ISR` or `Interrupt Service Routine` from this part). The `ISR` or `Interrupt Handler Routine` can be found in Interrupt Vector table that is located at fixed address in the memory. After the interrupt is handled processor resumes the interrupted process. At the boot/initialization time, the Linux kernel identifies all devices in the machine, and appropriate interrupt handlers are loaded into the interrupt table. As we saw in the previous parts, most exceptions are handled simply by the sending a [Unix signal](https://en.wikipedia.org/wiki/Unix_signal) to the interrupted process. That's why kernel is can handle an exception quickly. Unfortunately we can not use this approach for the external hardware interrupts, because often they arrive after (and sometimes long after) the process to which they are related has been suspended. So it would make no sense to send a Unix signal to the current process. External interrupt handling depends on the type of an interrupt:
|
||||
Interrupts are signal that are sent across [IRQ](https://en.wikipedia.org/wiki/Interrupt_request_%28PC_architecture%29) or `Interrupt Request Line` by a hardware or software. External hardware interrupts allow devices like keyboard, mouse and etc, to indicate that it needs attention of the processor. Once the processor receives the `Interrupt Request`, it will temporary stop execution of the running program and invoke special routine which depends on an interrupt. We already know that this routine is called interrupt handler (or how we will call it `ISR` or `Interrupt Service Routine` from this part). The `ISR` or `Interrupt Handler Routine` can be found in Interrupt Vector table that is located at fixed address in the memory. After the interrupt is handled processor resumes the interrupted process. At the boot/initialization time, the Linux kernel identifies all devices in the machine, and appropriate interrupt handlers are loaded into the interrupt table. As we saw in the previous parts, most exceptions are handled simply by the sending a [Unix signal](https://en.wikipedia.org/wiki/Unix_signal) to the interrupted process. That's how the kernel can handle an exception quickly. Unfortunately we can not use this approach for the external hardware interrupts, because often they arrive after (and sometimes long after) the process to which they are related has been suspended. So it would make no sense to send a Unix signal to the current process. External interrupt handling depends on the type of an interrupt:
|
||||
|
||||
* `I/O` interrupts;
|
||||
* Timer interrupts;
|
||||
@ -21,7 +21,7 @@ Generally, a handler of an `I/O` interrupt must be flexible enough to service se
|
||||
* Execute the interrupt service routine (next we will call it `ISR`) which is associated with the device;
|
||||
* Restore registers and return from an interrupt;
|
||||
|
||||
Ok, we know a little theory and now let's start with the `early_irq_init` function. The implementation of the `early_irq_init` function is in the [kernel/irq/irqdesc.c](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/kernel/irq/irqdesc.c). This function make early initialization of the `irq_desc` structure. The `irq_desc` structure is the foundation of interrupt management code in the Linux kernel. An array of this structure, which has the same name - `irq_desc`, keeps track of every interrupt request source in the Linux kernel. This structure defined in the [include/linux/irqdesc.h](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/include/linux/irqdesc.h) and as you can note it depends on the `CONFIG_SPARSE_IRQ` kernel configuration option. This kernel configuration option enables support for sparse irqs. The `irq_desc` structure contains many different files:
|
||||
Ok, we know a little theory and now let's start with the `early_irq_init` function. The implementation of the `early_irq_init` function is in the [kernel/irq/irqdesc.c](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/kernel/irq/irqdesc.c). This function make early initialization of the `irq_desc` structure. The `irq_desc` structure is the foundation of interrupt management code in the Linux kernel. An array of this structure, which has the same name - `irq_desc`, keeps track of every interrupt request source in the Linux kernel. This structure defined in the [include/linux/irqdesc.h](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/include/linux/irqdesc.h) and as you can note it depends on the `CONFIG_SPARSE_IRQ` kernel configuration option. This kernel configuration option enables support for sparse IRQs. The `irq_desc` structure contains many different fields:
|
||||
|
||||
* `irq_common_data` - per irq and chip data passed down to chip functions;
|
||||
* `status_use_accessors` - contains status of the interrupt source which is combination of the values from the `enum` from the [include/linux/irq.h](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/include/linux/irq.h) and different macros which are defined in the same source code file;
|
||||
@ -29,7 +29,7 @@ Ok, we know a little theory and now let's start with the `early_irq_init` functi
|
||||
* `handle_irq` - highlevel irq-events handler;
|
||||
* `action` - identifies the interrupt service routines to be invoked when the [IRQ](https://en.wikipedia.org/wiki/Interrupt_request_%28PC_architecture%29) occurs;
|
||||
* `irq_count` - counter of interrupt occurrences on the IRQ line;
|
||||
* `depth` - `0` if the IRQ line is enabled and a positive value if it has been disabled at least once;
|
||||
* `depth` - `0` if the IRQ line is enabled and a positive value if it has been disabled at least once;
|
||||
* `last_unhandled` - aging timer for unhandled count;
|
||||
* `irqs_unhandled` - count of the unhandled interrupts;
|
||||
* `lock` - a spin lock used to serialize the accesses to the `IRQ` descriptor;
|
||||
@ -75,10 +75,10 @@ As I already wrote, implementation of the `first_online_node` macro depends on t
|
||||
#if MAX_NUMNODES > 1
|
||||
#define first_online_node first_node(node_states[N_ONLINE])
|
||||
#else
|
||||
#define first_online_node 0
|
||||
#define first_online_node 0
|
||||
```
|
||||
|
||||
The `node_states` is the [enum](https://en.wikipedia.org/wiki/Enumerated_type) which defined in the [include/linux/nodemask.h](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/include/linux/nodemask.h) and represent the set of the states of a node. In our case we are searching an online node and it will be `0` if `MAX_NUMNODES` is one or zero. If the `MAX_NUMNODES` is greater than one, the `node_states[N_ONLINE]` will return `1` and the `first_node` macro will be expands to the call of the `__first_node` function which will return `minimal` or the first online node:
|
||||
The `node_states` is the [enum](https://en.wikipedia.org/wiki/Enumerated_type) which defined in the [include/linux/nodemask.h](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/include/linux/nodemask.h) and represent the set of the states of a node. In our case we are searching an online node and it will be `0` if `MAX_NUMNODES` is one or zero. If the `MAX_NUMNODES` is greater than one, the `node_states[N_ONLINE]` will return `1` and the `first_node` macro will be expanded to the call of the `__first_node` function which will return `minimal` or the first online node:
|
||||
|
||||
```C
|
||||
#define first_node(src) __first_node(&(src))
|
||||
@ -113,7 +113,7 @@ static void __init init_irq_default_affinity(void)
|
||||
#endif
|
||||
```
|
||||
|
||||
We know that when a hardware, such as disk controller or keyboard, needs attention from the processor, it throws an interrupt. The interrupt tells to the processor that something has happened and that the processor should interrupt current process and handle an incoming event. In order to prevent multiple devices from sending the same interrupts, the [IRQ](https://en.wikipedia.org/wiki/Interrupt_request_%28PC_architecture%29) system was established where each device in a computer system is assigned its own special IRQ so that its interrupts are unique. Linux kernel can assign certain `IRQs` to specific processors. This is known as `SMP IRQ affinity`, and it allows you control how your system will respond to various hardware events (that's why it has certain implementation only if the `CONFIG_SMP` kernel configuration option is set). After we allocated `irq_default_affinity` cpumask, we can see `printk` output:
|
||||
We know that when a hardware, such as disk controller or keyboard, needs attention from the processor, it throws an interrupt. The interrupt tells to the processor that something has happened and that the processor should interrupt current process and handle an incoming event. In order to prevent multiple devices from sending the same interrupts, the [IRQ](https://en.wikipedia.org/wiki/Interrupt_request_%28PC_architecture%29) system was established where each device in a computer system is assigned its own special IRQ so that its interrupts are unique. Linux kernel can assign certain `IRQs` to specific processors. This is known as `SMP IRQ affinity`, and it allows you to control how your system will respond to various hardware events (that's why it has certain implementation only if the `CONFIG_SMP` kernel configuration option is set). After we allocated `irq_default_affinity` cpumask, we can see `printk` output:
|
||||
|
||||
```C
|
||||
printk(KERN_INFO "NR_IRQS:%d\n", NR_IRQS);
|
||||
@ -189,7 +189,7 @@ struct irq_desc irq_desc[NR_IRQS] __cacheline_aligned_in_smp = {
|
||||
|
||||
The `irq_desc` is array of the `irq` descriptors. It has three already initialized fields:
|
||||
|
||||
* `handle_irq` - as I already wrote above, this field is the highlevel irq-event handler. In our case it initialized with the `handle_bad_irq` function that defined in the [kernel/irq/handle.c](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/kernel/irq/handle.c) source code file and handles spurious and unhandled irqs;
|
||||
* `handle_irq` - as I already wrote above, this field is the highlevel irq-event handler. In our case it initialized with the `handle_bad_irq` function that defined in the [kernel/irq/handle.c](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/kernel/irq/handle.c) source code file and handles spurious and unhandled IRQs;
|
||||
* `depth` - `0` if the IRQ line is enabled and a positive value if it has been disabled at least once;
|
||||
* `lock` - A spin lock used to serialize the accesses to the `IRQ` descriptor.
|
||||
|
||||
@ -258,7 +258,7 @@ irqd_set(&desc->irq_data, IRQD_IRQ_DISABLED);
|
||||
...
|
||||
```
|
||||
|
||||
In the next step we set the high level interrupt handlers to the `handle_bad_irq` which handles spurious and unhandled irqs (as the hardware stuff is not initialized yet, we set this handler), set `irq_desc.desc` to `1` which means that an `IRQ` is disabled, reset count of the unhandled interrupts and interrupts in general:
|
||||
In the next step we set the high level interrupt handlers to the `handle_bad_irq` which handles spurious and unhandled IRQs (as the hardware stuff is not initialized yet, we set this handler), set `irq_desc.desc` to `1` which means that an `IRQ` is disabled, reset count of the unhandled interrupts and interrupts in general:
|
||||
|
||||
```C
|
||||
...
|
||||
@ -294,14 +294,14 @@ static void desc_smp_init(struct irq_desc *desc, int node)
|
||||
#endif
|
||||
}
|
||||
```
|
||||
|
||||
|
||||
In the end of the `early_irq_init` function we return the return value of the `arch_early_irq_init` function:
|
||||
|
||||
```C
|
||||
return arch_early_irq_init();
|
||||
```
|
||||
|
||||
This function defined in the [kernel/apic/vector.c](https://github.com/torvalds/linux/blob/master/arch/x86/kernel/apic/vector.c) and contains only one call of the `arch_early_ioapic_init` function from the [kernel/apic/io_apic.c](https://github.com/torvalds/linux/blob/master/arch/x86/kernel/apic/io_apic.c). As we can understand from the `arch_early_ioapic_init` function's name, this function makes early initialization of the [I/O APIC](https://en.wikipedia.org/wiki/Advanced_Programmable_Interrupt_Controller). First of all it make a check of the number of the legacy interrupts with the call of the `nr_legacy_irqs` function. If we have no legacy interrupts with the [Intel 8259](https://en.wikipedia.org/wiki/Intel_8259) programmable interrupt controller we set `io_apic_irqs` to the `0xffffffffffffffff`:
|
||||
This function defined in the [kernel/apic/vector.c](https://github.com/torvalds/linux/blob/master/arch/x86/kernel/apic/vector.c) and contains only one call of the `arch_early_ioapic_init` function from the [kernel/apic/io_apic.c](https://github.com/torvalds/linux/blob/master/arch/x86/kernel/apic/io_apic.c). As we can understand from the `arch_early_ioapic_init` function's name, this function makes early initialization of the [I/O APIC](https://en.wikipedia.org/wiki/Advanced_Programmable_Interrupt_Controller). First of all it make a check of the number of the legacy interrupts with the call of the `nr_legacy_irqs` function. If we have no legacy interrupts with the [Intel 8259](https://en.wikipedia.org/wiki/Intel_8259) programmable interrupt controller we set `io_apic_irqs` to the `0xffffffffffffffff`:
|
||||
|
||||
```C
|
||||
if (!nr_legacy_irqs())
|
||||
@ -315,7 +315,7 @@ for_each_ioapic(i)
|
||||
alloc_ioapic_saved_registers(i);
|
||||
```
|
||||
|
||||
And in the end of the `arch_early_ioapic_init` function we are going through the all legacy irqs (from `IRQ0` to `IRQ15`) in the loop and allocate space for the `irq_cfg` which represents configuration of an irq on the given `NUMA` node:
|
||||
And in the end of the `arch_early_ioapic_init` function we are going through the all legacy IRQs (from `IRQ0` to `IRQ15`) in the loop and allocate space for the `irq_cfg` which represents configuration of an irq on the given `NUMA` node:
|
||||
|
||||
```C
|
||||
for (i = 0; i < nr_legacy_irqs(); i++) {
|
||||
@ -330,7 +330,7 @@ That's all.
|
||||
Sparse IRQs
|
||||
--------------------------------------------------------------------------------
|
||||
|
||||
We already saw in the beginning of this part that implementation of the `early_irq_init` function depends on the `CONFIG_SPARSE_IRQ` kernel configuration option. Previously we saw implementation of the `early_irq_init` function when the `CONFIG_SPARSE_IRQ` configuration option is not set, now let's look on the its implementation when this option is set. Implementation of this function very similar, but little differ. We can see the same definition of variables and call of the `init_irq_default_affinity` in the beginning of the `early_irq_init` function:
|
||||
We already saw in the beginning of this part that implementation of the `early_irq_init` function depends on the `CONFIG_SPARSE_IRQ` kernel configuration option. Previously we saw implementation of the `early_irq_init` function when the `CONFIG_SPARSE_IRQ` configuration option is not set, now let's look at its implementation when this option is set. Implementation of this function very similar, but little differ. We can see the same definition of variables and call of the `init_irq_default_affinity` in the beginning of the `early_irq_init` function:
|
||||
|
||||
```C
|
||||
#ifdef CONFIG_SPARSE_IRQ
|
||||
@ -356,7 +356,7 @@ But after this we can see the following call:
|
||||
initcnt = arch_probe_nr_irqs();
|
||||
```
|
||||
|
||||
The `arch_probe_nr_irqs` function defined in the [arch/x86/kernel/apic/vector.c](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/arch/x86/kernel/apic/vector.c) and calculates count of the pre-allocated irqs and update `nr_irqs` with its number. But stop. Why there are pre-allocated irqs? There is alternative form of interrupts called - [Message Signaled Interrupts](https://en.wikipedia.org/wiki/Message_Signaled_Interrupts) available in the [PCI](https://en.wikipedia.org/wiki/Conventional_PCI). Instead of assigning a fixed number of the interrupt request, the device is allowed to record a message at a particular address of RAM, in fact, the display on the [Local APIC](https://en.wikipedia.org/wiki/Advanced_Programmable_Interrupt_Controller#Integrated_local_APICs). `MSI` permits a device to allocate `1`, `2`, `4`, `8`, `16` or `32` interrupts and `MSI-X` permits a device to allocate up to `2048` interrupts. Now we know that irqs can be pre-allocated. More about `MSI` will be in a next part, but now let's look on the `arch_probe_nr_irqs` function. We can see the check which assign amount of the interrupt vectors for the each processor in the system to the `nr_irqs` if it is greater and calculate the `nr` which represents number of `MSI` interrupts:
|
||||
The `arch_probe_nr_irqs` function defined in the [arch/x86/kernel/apic/vector.c](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/arch/x86/kernel/apic/vector.c) and calculates count of the pre-allocated IRQs and update `nr_irqs` with this number. But stop. Why are there pre-allocated IRQs? There is alternative form of interrupts called - [Message Signaled Interrupts](https://en.wikipedia.org/wiki/Message_Signaled_Interrupts) available in the [PCI](https://en.wikipedia.org/wiki/Conventional_PCI). Instead of assigning a fixed number of the interrupt request, the device is allowed to record a message at a particular address of RAM, in fact, the display on the [Local APIC](https://en.wikipedia.org/wiki/Advanced_Programmable_Interrupt_Controller#Integrated_local_APICs). `MSI` permits a device to allocate `1`, `2`, `4`, `8`, `16` or `32` interrupts and `MSI-X` permits a device to allocate up to `2048` interrupts. Now we know that IRQs can be pre-allocated. More about `MSI` will be in a next part, but now let's look on the `arch_probe_nr_irqs` function. We can see the check which assign amount of the interrupt vectors for the each processor in the system to the `nr_irqs` if it is greater and calculate the `nr` which represents number of `MSI` interrupts:
|
||||
|
||||
```C
|
||||
int nr_irqs = NR_IRQS;
|
||||
@ -367,7 +367,7 @@ if (nr_irqs > (NR_VECTORS * nr_cpu_ids))
|
||||
nr = (gsi_top + nr_legacy_irqs()) + 8 * nr_cpu_ids;
|
||||
```
|
||||
|
||||
Take a look on the `gsi_top` variable. Each `APIC` is identified with its own `ID` and with the offset where its `IRQ` starts. It is called `GSI` base or `Global System Interrupt` base. So the `gsi_top` represents it. We get the `Global System Interrupt` base from the [MultiProcessor Configuration Table](https://en.wikipedia.org/wiki/MultiProcessor_Specification) table (you can remember that we have parsed this table in the sixth [part](https://0xax.gitbook.io/linux-insides/summary/initialization/linux-initialization-6) of the Linux Kernel initialization process chapter).
|
||||
Take a look on the `gsi_top` variable. Each `APIC` is identified with its own `ID` and with the offset where its `IRQ` starts. It is called `GSI` base or `Global System Interrupt` base. So the `gsi_top` represents it. We get the `Global System Interrupt` base from the [MultiProcessor Configuration Table](https://en.wikipedia.org/wiki/MultiProcessor_Specification) table (you can remember that we have parsed this table in the sixth [part](https://0xax.gitbook.io/linux-insides/summary/initialization/linux-initialization-6) of the Linux kernel initialization process chapter).
|
||||
|
||||
After this we update the `nr` depends on the value of the `gsi_top`:
|
||||
|
||||
@ -380,7 +380,7 @@ After this we update the `nr` depends on the value of the `gsi_top`:
|
||||
#endif
|
||||
```
|
||||
|
||||
Update the `nr_irqs` if it less than `nr` and return the number of the legacy irqs:
|
||||
Update the `nr_irqs` if it less than `nr` and return the number of the legacy IRQs:
|
||||
|
||||
```C
|
||||
if (nr < nr_irqs)
|
||||
|
||||
@ -113,10 +113,10 @@ In the end of the `init_IRQ` function we can see the call of the following funct
|
||||
x86_init.irqs.intr_init();
|
||||
```
|
||||
|
||||
from the [arch/x86/kernel/x86_init.c](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/arch/x86/kernel/x86_init.c) source code file. If you have read [chapter](https://0xax.gitbook.io/linux-insides/summary/initialization) about the Linux kernel initialization process, you can remember the `x86_init` structure. This structure contains a couple of files which are points to the function related to the platform setup (`x86_64` in our case), for example `resources` - related with the memory resources, `mpparse` - related with the parsing of the [MultiProcessor Configuration Table](https://en.wikipedia.org/wiki/MultiProcessor_Specification) table and etc.). As we can see the `x86_init` also contains the `irqs` field which contains three following fields:
|
||||
from the [arch/x86/kernel/x86_init.c](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/arch/x86/kernel/x86_init.c) source code file. If you have read [chapter](https://0xax.gitbook.io/linux-insides/summary/initialization) about the Linux kernel initialization process, you can remember the `x86_init` structure. This structure contains a couple of files which point to the function related to the platform setup (`x86_64` in our case), for example `resources` - related with the memory resources, `mpparse` - related with the parsing of the [MultiProcessor Configuration Table](https://en.wikipedia.org/wiki/MultiProcessor_Specification) table, etc.). As we can see the `x86_init` also contains the `irqs` field which contains the three following fields:
|
||||
|
||||
```C
|
||||
struct x86_init_ops x86_init __initdata
|
||||
struct x86_init_ops x86_init __initdata
|
||||
{
|
||||
...
|
||||
...
|
||||
@ -132,7 +132,7 @@ struct x86_init_ops x86_init __initdata
|
||||
}
|
||||
```
|
||||
|
||||
Now, we are interesting in the `native_init_IRQ`. As we can note, the name of the `native_init_IRQ` function contains the `native_` prefix which means that this function is architecture-specific. It defined in the [arch/x86/kernel/irqinit.c](https://github.com/torvalds/linux/blob/master/arch/x86/kernel/irqinit.c) and executes general initialization of the [Local APIC](https://en.wikipedia.org/wiki/Advanced_Programmable_Interrupt_Controller#Integrated_local_APICs) and initialization of the [ISA](https://en.wikipedia.org/wiki/Industry_Standard_Architecture) irqs. Let's look on the implementation of the `native_init_IRQ` function and will try to understand what occurs there. The `native_init_IRQ` function starts from the execution of the following function:
|
||||
Now, we are interesting in the `native_init_IRQ`. As we can note, the name of the `native_init_IRQ` function contains the `native_` prefix which means that this function is architecture-specific. It defined in the [arch/x86/kernel/irqinit.c](https://github.com/torvalds/linux/blob/master/arch/x86/kernel/irqinit.c) and executes general initialization of the [Local APIC](https://en.wikipedia.org/wiki/Advanced_Programmable_Interrupt_Controller#Integrated_local_APICs) and initialization of the [ISA](https://en.wikipedia.org/wiki/Industry_Standard_Architecture) irqs. Let's look at the implementation of the `native_init_IRQ` function and try to understand what occurs there. The `native_init_IRQ` function starts from the execution of the following function:
|
||||
|
||||
```C
|
||||
x86_init.irqs.pre_vector_init();
|
||||
@ -155,21 +155,21 @@ The `irq_chip` structure defined in the [include/linux/irq.h](https://github.com
|
||||
|
||||
```C
|
||||
$ cat /proc/interrupts
|
||||
CPU0 CPU1 CPU2 CPU3 CPU4 CPU5 CPU6 CPU7
|
||||
CPU0 CPU1 CPU2 CPU3 CPU4 CPU5 CPU6 CPU7
|
||||
0: 16 0 0 0 0 0 0 0 IO-APIC 2-edge timer
|
||||
1: 2 0 0 0 0 0 0 0 IO-APIC 1-edge i8042
|
||||
8: 1 0 0 0 0 0 0 0 IO-APIC 8-edge rtc0
|
||||
```
|
||||
|
||||
look on the last column;
|
||||
look at the last column;
|
||||
|
||||
* `(*irq_mask)(struct irq_data *data)` - mask an interrupt source;
|
||||
* `(*irq_ack)(struct irq_data *data)` - start of a new interrupt;
|
||||
* `(*irq_startup)(struct irq_data *data)` - start up the interrupt;
|
||||
* `(*irq_shutdown)(struct irq_data *data)` - shutdown the interrupt
|
||||
* and etc.
|
||||
* etc.
|
||||
|
||||
fields. Note that the `irq_data` structure represents set of the per irq chip data passed down to chip functions. It contains `mask` - precomputed bitmask for accessing the chip registers, `irq` - interrupt number, `hwirq` - hardware interrupt number, local to the interrupt domain chip low level interrupt hardware access and etc.
|
||||
fields. Note that the `irq_data` structure represents set of the per irq chip data passed down to chip functions. It contains `mask` - precomputed bitmask for accessing the chip registers, `irq` - interrupt number, `hwirq` - hardware interrupt number, local to the interrupt domain chip low level interrupt hardware access, etc.
|
||||
|
||||
After this depends on the `CONFIG_X86_64` and `CONFIG_X86_LOCAL_APIC` kernel configuration option call the `init_bsp_APIC` function from the [arch/x86/kernel/apic/apic.c](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/arch/x86/kernel/apic/apic.c):
|
||||
|
||||
@ -186,7 +186,7 @@ if (smp_found_config || !cpu_has_apic)
|
||||
return;
|
||||
```
|
||||
|
||||
In other way we return from this function. In the next step we call the `clear_local_APIC` function from the same source code file that shutdowns the local `APIC` (more about it will be in the chapter about the `Advanced Programmable Interrupt Controller`) and enable `APIC` of the first processor by the setting `unsigned int value` to the `APIC_SPIV_APIC_ENABLED`:
|
||||
Otherwise, we return from this function. In the next step we call the `clear_local_APIC` function from the same source code file that shuts down the local `APIC` (more on it in the `Advanced Programmable Interrupt Controller` chapter) and enable `APIC` of the first processor by the setting `unsigned int value` to the `APIC_SPIV_APIC_ENABLED`:
|
||||
|
||||
```C
|
||||
value = apic_read(APIC_SPIV);
|
||||
@ -200,11 +200,11 @@ and writing it with the help of the `apic_write` function:
|
||||
apic_write(APIC_SPIV, value);
|
||||
```
|
||||
|
||||
After we have enabled `APIC` for the bootstrap processor, we return to the `init_ISA_irqs` function and in the next step we initialize legacy `Programmable Interrupt Controller` and set the legacy chip and handler for the each legacy irq:
|
||||
After we have enabled `APIC` for the bootstrap processor, we return to the `init_ISA_irqs` function and in the next step we initialize legacy `Programmable Interrupt Controller` and set the legacy chip and handler for each legacy irq:
|
||||
|
||||
```C
|
||||
legacy_pic->init(0);
|
||||
|
||||
|
||||
for (i = 0; i < nr_legacy_irqs(); i++)
|
||||
irq_set_chip_and_handler(i, chip, handle_level_irq);
|
||||
```
|
||||
@ -229,7 +229,7 @@ struct legacy_pic default_legacy_pic = {
|
||||
}
|
||||
```
|
||||
|
||||
The `init_8259A` function defined in the same source code file and executes initialization of the [Intel 8259](https://en.wikipedia.org/wiki/Intel_8259) ``Programmable Interrupt Controller` (more about it will be in the separate chapter about `Programmable Interrupt Controllers` and `APIC`).
|
||||
The `init_8259A` function defined in the same source code file and executes initialization of the [Intel 8259](https://en.wikipedia.org/wiki/Intel_8259) `Programmable Interrupt Controller` (more about it will be in the separate chapter about `Programmable Interrupt Controllers` and `APIC`).
|
||||
|
||||
Now we can return to the `native_init_IRQ` function, after the `init_ISA_irqs` function finished its work. The next step is the call of the `apic_intr_init` function that allocates special interrupt gates which are used by the [SMP](https://en.wikipedia.org/wiki/Symmetric_multiprocessing) architecture for the [Inter-processor interrupt](https://en.wikipedia.org/wiki/Inter-processor_interrupt). The `alloc_intr_gate` macro from the [arch/x86/include/asm/desc.h](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/arch/x86/include/asm/desc.h) used for the interrupt descriptor allocation:
|
||||
|
||||
@ -253,7 +253,7 @@ if (!test_bit(vector, used_vectors)) {
|
||||
}
|
||||
```
|
||||
|
||||
We already saw the `set_bit` macro, now let's look on the `test_bit` and the `first_system_vector`. The first `test_bit` macro defined in the [arch/x86/include/asm/bitops.h](https://github.com/torvalds/linux/blob/master/arch/x86/include/asm/bitops.h) and looks like this:
|
||||
We already saw the `set_bit` macro, now let's look at the `test_bit` and the `first_system_vector`. The first `test_bit` macro defined in the [arch/x86/include/asm/bitops.h](https://github.com/torvalds/linux/blob/master/arch/x86/include/asm/bitops.h) and looks like this:
|
||||
|
||||
```C
|
||||
#define test_bit(nr, addr) \
|
||||
@ -262,7 +262,7 @@ We already saw the `set_bit` macro, now let's look on the `test_bit` and the `fi
|
||||
: variable_test_bit((nr), (addr)))
|
||||
```
|
||||
|
||||
We can see the [ternary operator](https://en.wikipedia.org/wiki/Ternary_operation) here make a test with the [gcc](https://en.wikipedia.org/wiki/GNU_Compiler_Collection) built-in function `__builtin_constant_p` tests that given vector number (`nr`) is known at compile time. If you're feeling misunderstanding of the `__builtin_constant_p`, we can make simple test:
|
||||
We can see the [ternary operator](https://en.wikipedia.org/wiki/Ternary_operation) here makes a test with the [gcc](https://en.wikipedia.org/wiki/GNU_Compiler_Collection) built-in function `__builtin_constant_p` tests that given vector number (`nr`) is known at compile time. If you're feeling misunderstanding of the `__builtin_constant_p`, we can make simple test:
|
||||
|
||||
```C
|
||||
#include <stdio.h>
|
||||
@ -279,7 +279,7 @@ int main() {
|
||||
}
|
||||
```
|
||||
|
||||
and look on the result:
|
||||
and look at the result:
|
||||
|
||||
```
|
||||
$ gcc test.c -o test
|
||||
@ -289,7 +289,7 @@ __builtin_constant_p(PREDEFINED_VAL) is 1
|
||||
__builtin_constant_p(100) is 1
|
||||
```
|
||||
|
||||
Now I think it must be clear for you. Let's get back to the `test_bit` macro. If the `__builtin_constant_p` will return non-zero, we call `constant_test_bit` function:
|
||||
Now I think it must be clear for you. Let's get back to the `test_bit` macro. If the `__builtin_constant_p` returns non-zero, we call `constant_test_bit` function:
|
||||
|
||||
```C
|
||||
static inline int constant_test_bit(int nr, const void *addr)
|
||||
@ -313,7 +313,7 @@ static inline int variable_test_bit(int nr, const void *addr)
|
||||
}
|
||||
```
|
||||
|
||||
What's the difference between two these functions and why do we need in two different functions for the same purpose? As you already can guess main purpose is optimization. If we will write simple example with these functions:
|
||||
What's the difference between two these functions and why do we need in two different functions for the same purpose? As you already can guess main purpose is optimization. If we write simple example with these functions:
|
||||
|
||||
```C
|
||||
#define CONST 25
|
||||
@ -326,7 +326,7 @@ int main() {
|
||||
}
|
||||
```
|
||||
|
||||
and will look on the assembly output of our example we will see following assembly code:
|
||||
and will look at the assembly output of our example we will see following assembly code:
|
||||
|
||||
```assembly
|
||||
pushq %rbp
|
||||
@ -351,10 +351,10 @@ movl %eax, %edi
|
||||
call variable_test_bit
|
||||
```
|
||||
|
||||
for the `variable_test_bit`. These two code listings starts with the same part, first of all we save base of the current stack frame in the `%rbp` register. But after this code for both examples is different. In the first example we put `$268435456` (here the `$268435456` is our second parameter - `0x10000000`) to the `esi` and `$25` (our first parameter) to the `edi` register and call `constant_test_bit`. We put function parameters to the `esi` and `edi` registers because as we are learning Linux kernel for the `x86_64` architecture we use `System V AMD64 ABI` [calling convention](https://en.wikipedia.org/wiki/X86_calling_conventions). All is pretty simple. When we are using predefined constant, the compiler can just substitute its value. Now let's look on the second part. As you can see here, the compiler can not substitute value from the `nr` variable. In this case compiler must calculate its offset on the program's [stack frame](https://en.wikipedia.org/wiki/Call_stack). We subtract `16` from the `rsp` register to allocate stack for the local variables data and put the `$24` (value of the `nr` variable) to the `rbp` with offset `-4`. Our stack frame will be like this:
|
||||
for the `variable_test_bit`. These two code listings starts with the same part, first of all we save base of the current stack frame in the `%rbp` register. But after this code for both examples is different. In the first example we put `$268435456` (here the `$268435456` is our second parameter - `0x10000000`) to the `esi` and `$25` (our first parameter) to the `edi` register and call `constant_test_bit`. We put function parameters to the `esi` and `edi` registers because as we are learning Linux kernel for the `x86_64` architecture we use `System V AMD64 ABI` [calling convention](https://en.wikipedia.org/wiki/X86_calling_conventions). All is pretty simple. When we are using predefined constant, the compiler can just substitute its value. Now let's look at the second part. As you can see here, the compiler can not substitute value from the `nr` variable. In this case compiler must calculate its offset on the program's [stack frame](https://en.wikipedia.org/wiki/Call_stack). We subtract `16` from the `rsp` register to allocate stack for the local variables data and put the `$24` (value of the `nr` variable) to the `rbp` with offset `-4`. Our stack frame will be like this:
|
||||
|
||||
```
|
||||
<- stack grows
|
||||
<- stack grows
|
||||
|
||||
%[rbp]
|
||||
|
|
||||
@ -367,7 +367,7 @@ for the `variable_test_bit`. These two code listings starts with the same part,
|
||||
%[rsp]
|
||||
```
|
||||
|
||||
After this we put this value to the `eax`, so `eax` register now contains value of the `nr`. In the end we do the same that in the first example, we put the `$268435456` (the first parameter of the `variable_test_bit` function) and the value of the `eax` (value of `nr`) to the `edi` register (the second parameter of the `variable_test_bit function`).
|
||||
After this we put this value to the `eax`, so `eax` register now contains value of the `nr`. In the end we do the same that in the first example, we put the `$268435456` (the first parameter of the `variable_test_bit` function) and the value of the `eax` (value of `nr`) to the `edi` register (the second parameter of the `variable_test_bit function`).
|
||||
|
||||
The next step after the `apic_intr_init` function will finish its work is the setting interrupt gates from the `FIRST_EXTERNAL_VECTOR` or `0x20` up to `0x100`:
|
||||
|
||||
@ -408,7 +408,7 @@ if (!acpi_ioapic && !of_ioapic && nr_legacy_irqs())
|
||||
setup_irq(2, &irq2);
|
||||
```
|
||||
|
||||
First of all let's deal with the condition. The `acpi_ioapic` variable represents existence of [I/O APIC](https://en.wikipedia.org/wiki/Advanced_Programmable_Interrupt_Controller#I.2FO_APICs). It defined in the [arch/x86/kernel/acpi/boot.c](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/arch/x86/kernel/acpi/boot.c). This variable set in the `acpi_set_irq_model_ioapic` function that called during the processing `Multiple APIC Description Table`. This occurs during initialization of the architecture-specific stuff in the [arch/x86/kernel/setup.c](https://github.com/torvalds/linux/blob/master/arch/x86/kernel/setup.c) (more about it we will know in the other chapter about [APIC](https://en.wikipedia.org/wiki/Advanced_Programmable_Interrupt_Controller)). Note that the value of the `acpi_ioapic` variable depends on the `CONFIG_ACPI` and `CONFIG_X86_LOCAL_APIC` Linux kernel configuration options. If these options did not set, this variable will be just zero:
|
||||
First of all let's deal with the condition. The `acpi_ioapic` variable represents existence of [I/O APIC](https://en.wikipedia.org/wiki/Advanced_Programmable_Interrupt_Controller#I.2FO_APICs). It defined in the [arch/x86/kernel/acpi/boot.c](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/arch/x86/kernel/acpi/boot.c). This variable set in the `acpi_set_irq_model_ioapic` function that called during the processing `Multiple APIC Description Table`. This occurs during initialization of the architecture-specific stuff in the [arch/x86/kernel/setup.c](https://github.com/torvalds/linux/blob/master/arch/x86/kernel/setup.c) (more about it we will know in the other chapter about [APIC](https://en.wikipedia.org/wiki/Advanced_Programmable_Interrupt_Controller)). Note that the value of the `acpi_ioapic` variable depends on the `CONFIG_ACPI` and `CONFIG_X86_LOCAL_APIC` Linux kernel configuration options. If these options were not set, this variable will be just zero:
|
||||
|
||||
```C
|
||||
#define acpi_ioapic 0
|
||||
@ -430,7 +430,7 @@ extern int of_ioapic;
|
||||
#endif
|
||||
```
|
||||
|
||||
If the condition will return non-zero value we call the:
|
||||
If the condition returns non-zero value we call the:
|
||||
|
||||
```C
|
||||
setup_irq(2, &irq2);
|
||||
@ -465,7 +465,7 @@ Some time ago interrupt controller consisted of two chips and one was connected
|
||||
* `IRQ 6` - drive controller;
|
||||
* `IRQ 7` - `LPT1`.
|
||||
|
||||
The `setup_irq` function defined in the [kernel/irq/manage.c](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/kernel/irq/manage.c) and takes two parameters:
|
||||
The `setup_irq` function is defined in the [kernel/irq/manage.c](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/kernel/irq/manage.c) and takes two parameters:
|
||||
|
||||
* vector number of an interrupt;
|
||||
* `irqaction` structure related with an interrupt.
|
||||
@ -476,7 +476,7 @@ This function initializes interrupt descriptor from the given vector number at t
|
||||
struct irq_desc *desc = irq_to_desc(irq);
|
||||
```
|
||||
|
||||
And call the `__setup_irq` function that setups given interrupt:
|
||||
And call the `__setup_irq` function that sets up given interrupt:
|
||||
|
||||
```C
|
||||
chip_bus_lock(desc);
|
||||
@ -485,7 +485,7 @@ chip_bus_sync_unlock(desc);
|
||||
return retval;
|
||||
```
|
||||
|
||||
Note that the interrupt descriptor is locked during `__setup_irq` function will work. The `__setup_irq` function makes many different things: It creates a handler thread when a thread function is supplied and the interrupt does not nest into another interrupt thread, sets the flags of the chip, fills the `irqaction` structure and many many more.
|
||||
Note that the interrupt descriptor is locked during `__setup_irq` function will work. The `__setup_irq` function does many different things: it creates a handler thread when a thread function is supplied and the interrupt does not nest into another interrupt thread, sets the flags of the chip, fills the `irqaction` structure and many many more.
|
||||
|
||||
All of the above it creates `/prov/vector_number` directory and fills it, but if you are using modern computer all values will be zero there:
|
||||
|
||||
@ -493,16 +493,16 @@ All of the above it creates `/prov/vector_number` directory and fills it, but if
|
||||
$ cat /proc/irq/2/node
|
||||
0
|
||||
|
||||
$cat /proc/irq/2/affinity_hint
|
||||
$cat /proc/irq/2/affinity_hint
|
||||
00
|
||||
|
||||
cat /proc/irq/2/spurious
|
||||
cat /proc/irq/2/spurious
|
||||
count 0
|
||||
unhandled 0
|
||||
last_unhandled 0 ms
|
||||
```
|
||||
|
||||
because probably `APIC` handles interrupts on the our machine.
|
||||
because probably `APIC` handles interrupts on the machine.
|
||||
|
||||
That's all.
|
||||
|
||||
|
||||
@ -16,7 +16,7 @@ As you can understand, it is almost impossible to make so that both characterist
|
||||
* Top half;
|
||||
* Bottom half;
|
||||
|
||||
In the past there was one way to defer interrupt handling in Linux kernel. And it was called: `the bottom half` of the processor, but now it is already not actual. Now this term has remained as a common noun referring to all the different ways of organizing deferred processing of an interrupt.The deferred processing of an interrupt suggests that some of the actions for an interrupt may be postponed to a later execution when the system will be less loaded. As you can suggest, an interrupt handler can do large amount of work that is impermissible as it executes in the context where interrupts are disabled. That's why processing of an interrupt can be split on two different parts. In the first part, the main handler of an interrupt does only minimal and the most important job. After this it schedules the second part and finishes its work. When the system is less busy and context of the processor allows to handle interrupts, the second part starts its work and finishes to process remaining part of a deferred interrupt.
|
||||
In the past there was one way to defer interrupt handling in Linux kernel. And it was called: `the bottom half` of the processor, but now it is already not actual. Now this term has remained as a common noun referring to all the different ways of organizing deferred processing of an interrupt.The deferred processing of an interrupt suggests that some of the actions for an interrupt may be postponed to a later execution when the system will be less loaded. As you can suggest, an interrupt handler can do large amount of work that is impermissible as it executes in the context where interrupts are disabled. That's why processing of an interrupt can be split in two different parts. In the first part, the main handler of an interrupt does only minimal and the most important job. After this it schedules the second part and finishes its work. When the system is less busy and context of the processor allows to handle interrupts, the second part starts its work and finishes to process remaining part of a deferred interrupt.
|
||||
|
||||
There are three types of `deferred interrupts` in the Linux kernel:
|
||||
|
||||
@ -101,8 +101,8 @@ const char * const softirq_to_name[NR_SOFTIRQS] = {
|
||||
Or we can see it in the output of the `/proc/softirqs`:
|
||||
|
||||
```
|
||||
~$ cat /proc/softirqs
|
||||
CPU0 CPU1 CPU2 CPU3 CPU4 CPU5 CPU6 CPU7
|
||||
~$ cat /proc/softirqs
|
||||
CPU0 CPU1 CPU2 CPU3 CPU4 CPU5 CPU6 CPU7
|
||||
HI: 5 0 0 0 0 0 0 0
|
||||
TIMER: 332519 310498 289555 272913 282535 279467 282895 270979
|
||||
NET_TX: 2320 0 0 2 1 1 0 0
|
||||
@ -139,7 +139,7 @@ void raise_softirq(unsigned int nr)
|
||||
|
||||
Here we can see the call of the `raise_softirq_irqoff` function between the `local_irq_save` and the `local_irq_restore` macros. The `local_irq_save` defined in the [include/linux/irqflags.h](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/include/linux/irqflags.h) header file and saves the state of the [IF](https://en.wikipedia.org/wiki/Interrupt_flag) flag of the [eflags](https://en.wikipedia.org/wiki/FLAGS_register) register and disables interrupts on the local processor. The `local_irq_restore` macro defined in the same header file and does the opposite thing: restores the `interrupt flag` and enables interrupts. We disable interrupts here because a `softirq` interrupt runs in the interrupt context and that one softirq (and no others) will be run.
|
||||
|
||||
The `raise_softirq_irqoff` function marks the softirq as deffered by setting the bit corresponding to the given index `nr` in the `softirq` bit mask (`__softirq_pending`) of the local processor. It does it with the help of the:
|
||||
The `raise_softirq_irqoff` function marks the softirq as deferred by setting the bit corresponding to the given index `nr` in the `softirq` bit mask (`__softirq_pending`) of the local processor. It does it with the help of the:
|
||||
|
||||
```C
|
||||
__raise_softirq_irqoff(nr);
|
||||
@ -186,7 +186,7 @@ if (pending) {
|
||||
--max_restart)
|
||||
goto restart;
|
||||
}
|
||||
...
|
||||
...
|
||||
```
|
||||
|
||||
Checks of the existence of the deferred interrupts are performed periodically. There are several points where these checks occur. The main point is the call of the `do_IRQ` function defined in [arch/x86/kernel/irq.c](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/arch/x86/kernel/irq.c), which provides the main means for actual interrupt processing in the Linux kernel. When `do_IRQ` finishes handling an interrupt, it calls the `exiting_irq` function from the [arch/x86/include/asm/apic.h](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/arch/x86/include/asm/apic.h) that expands to the call of the `irq_exit` function. `irq_exit` checks for deferred interrupts and the current context and calls the `invoke_softirq` function:
|
||||
@ -284,7 +284,7 @@ As we can see this structure contains five fields, they are:
|
||||
* Parameter of the callback.
|
||||
|
||||
In our case, we set only for initialize only two arrays of tasklets in the `softirq_init` function: the `tasklet_vec` and the `tasklet_hi_vec`. Tasklets and high-priority tasklets are stored in the `tasklet_vec` and `tasklet_hi_vec` arrays, respectively. So, we have initialized these arrays and now we can see two calls of the `open_softirq` function that is defined in the [kernel/softirq.c](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/kernel/softirq.c) source code file:
|
||||
|
||||
|
||||
```C
|
||||
open_softirq(TASKLET_SOFTIRQ, tasklet_action);
|
||||
open_softirq(HI_SOFTIRQ, tasklet_hi_action);
|
||||
@ -292,7 +292,7 @@ open_softirq(HI_SOFTIRQ, tasklet_hi_action);
|
||||
|
||||
at the end of the `softirq_init` function. The main purpose of the `open_softirq` function is the initialization of `softirq`. Let's look on the implementation of the `open_softirq` function.
|
||||
|
||||
, in our case they are: `tasklet_action` and the `tasklet_hi_action` or the `softirq` function associated with the `HI_SOFTIRQ` softirq is named `tasklet_hi_action` and `softirq` function associated with the `TASKLET_SOFTIRQ` is named `tasklet_action`. The Linux kernel provides API for the manipulating of `tasklets`. First of all it is the `tasklet_init` function that takes `tasklet_struct`, function and parameter for it and initializes the given `tasklet_struct` with the given data:
|
||||
In our case they are: `tasklet_action` and the `tasklet_hi_action` or the `softirq` function associated with the `HI_SOFTIRQ` softirq is named `tasklet_hi_action` and `softirq` function associated with the `TASKLET_SOFTIRQ` is named `tasklet_action`. The Linux kernel provides API for the manipulating of `tasklets`. First of all it is the `tasklet_init` function that takes `tasklet_struct`, function and parameter for it and initializes the given `tasklet_struct` with the given data:
|
||||
|
||||
```C
|
||||
void tasklet_init(struct tasklet_struct *t,
|
||||
@ -310,7 +310,7 @@ There are additional methods to initialize a tasklet statically with the two fol
|
||||
|
||||
```C
|
||||
DECLARE_TASKLET(name, func, data);
|
||||
DECLARE_TASKLET_DISABLED(name, func, data);
|
||||
DECLARE_TASKLET_DISABLED(name, func, data);
|
||||
```
|
||||
|
||||
The Linux kernel provides three following functions to mark a tasklet as ready to run:
|
||||
@ -368,7 +368,7 @@ static void tasklet_action(struct softirq_action *a)
|
||||
}
|
||||
```
|
||||
|
||||
In the beginning of the `tasklet_action` function, we disable interrupts for the local processor with the help of the `local_irq_disable` macro (you can read about this macro in the second [part](https://0xax.gitbook.io/linux-insides/summary/interrupts/linux-interrupts-2) of this chapter). In the next step, we take a head of the list that contains tasklets with normal priority and set this per-cpu list to `NULL` because all tasklets must be executed in a generally way. After this we enable interrupts for the local processor and go through the list of tasklets in the loop. In every iteration of the loop we call the `tasklet_trylock` function for the given tasklet that updates state of the given tasklet on `TASKLET_STATE_RUN`:
|
||||
In the beginning of the `tasklet_action` function, we disable interrupts for the local processor with the help of the `local_irq_disable` macro (you can read about this macro in the second [part](https://0xax.gitbook.io/linux-insides/summary/interrupts/linux-interrupts-2) of this chapter). In the next step, we take a head of the list that contains tasklets with normal priority and set this per-cpu list to `NULL` because all tasklets must be executed in a general way. After this we enable interrupts for the local processor and go through the list of tasklets in the loop. In every iteration of the loop we call the `tasklet_trylock` function for the given tasklet that updates state of the given tasklet on `TASKLET_STATE_RUN`:
|
||||
|
||||
```C
|
||||
static inline int tasklet_trylock(struct tasklet_struct *t)
|
||||
|
||||
437
LICENSE.md
Normal file
437
LICENSE.md
Normal file
@ -0,0 +1,437 @@
|
||||
Attribution-NonCommercial-ShareAlike 4.0 International
|
||||
|
||||
=======================================================================
|
||||
|
||||
Creative Commons Corporation ("Creative Commons") is not a law firm and
|
||||
does not provide legal services or legal advice. Distribution of
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||||
Creative Commons public licenses does not create a lawyer-client or
|
||||
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|
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|
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|
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|
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|
||||
Using Creative Commons Public Licenses
|
||||
|
||||
Creative Commons public licenses provide a standard set of terms and
|
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conditions that creators and other rights holders may use to share
|
||||
original works of authorship and other material subject to copyright
|
||||
and certain other rights specified in the public license below. The
|
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|
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|
||||
Considerations for licensors: Our public licenses are
|
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intended for use by those authorized to give the public
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|
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Licensors should also secure all rights necessary before
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Considerations for the public: By using one of our public
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Although not required by our licenses, you are encouraged to
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for the public:
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wiki.creativecommons.org/Considerations_for_licensees
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=======================================================================
|
||||
|
||||
Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International
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Public License
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|
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By exercising the Licensed Rights (defined below), You accept and agree
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Section 1 -- Definitions.
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a. Adapted Material means material subject to Copyright and Similar
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b. Adapter's License means the license You apply to Your Copyright
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|
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|
||||
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|
||||
|
||||
e. Effective Technological Measures means those measures that, in the
|
||||
absence of proper authority, may not be circumvented under laws
|
||||
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|
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|
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|
||||
|
||||
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|
||||
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|
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|
||||
|
||||
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|
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|
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|
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|
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|
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|
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|
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|
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|
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|
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|
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|
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|
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m. Sui Generis Database Rights means rights other than copyright
|
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|
||||
the Council of 11 March 1996 on the legal protection of databases,
|
||||
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|
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|
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|
||||
n. You means the individual or entity exercising the Licensed Rights
|
||||
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|
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|
||||
|
||||
Section 2 -- Scope.
|
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|
||||
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|
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|
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|
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the Licensor hereby grants You a worldwide, royalty-free,
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|
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|
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|
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|
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|
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|
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|
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|
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|
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|
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|
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|
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|
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|
||||
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||||
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|
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|
||||
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|
||||
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|
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|
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|
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|
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|
||||
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|
||||
|
||||
Your exercise of the Licensed Rights is expressly made subject to the
|
||||
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|
||||
|
||||
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|
||||
|
||||
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|
||||
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|
||||
|
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|
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|
||||
|
||||
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|
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|
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|
||||
the Licensor (including by pseudonym if
|
||||
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|
||||
|
||||
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|
||||
|
||||
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|
||||
|
||||
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|
||||
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|
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|
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|
||||
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|
||||
|
||||
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|
||||
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|
||||
|
||||
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|
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|
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|
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|
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|
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|
||||
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|
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|
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|
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|
||||
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|
||||
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|
||||
|
||||
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|
||||
|
||||
In addition to the conditions in Section 3(a), if You Share
|
||||
Adapted Material You produce, the following conditions also apply.
|
||||
|
||||
1. The Adapter's License You apply must be a Creative Commons
|
||||
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|
||||
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|
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|
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|
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|
||||
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|
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|
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|
||||
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|
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|
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|
||||
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|
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|
||||
|
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|
||||
|
||||
Where the Licensed Rights include Sui Generis Database Rights that
|
||||
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|
||||
|
||||
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|
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|
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|
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|
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|
||||
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|
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|
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|
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|
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|
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|
||||
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|
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|
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|
||||
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|
||||
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|
||||
|
||||
|
||||
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|
||||
|
||||
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|
||||
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|
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|
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|
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|
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|
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|
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|
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|
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|
||||
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|
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|
||||
|
||||
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|
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|
||||
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|
||||
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|
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|
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|
||||
|
||||
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|
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|
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|
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|
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|
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|
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|
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|
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|
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|
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|
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|
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|
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|
||||
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|
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|
||||
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|
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|
||||
|
||||
|
||||
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|
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|
||||
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|
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|
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|
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|
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|
||||
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|
||||
|
||||
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|
||||
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|
||||
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|
||||
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|
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|
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|
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|
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|
||||
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|
||||
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|
||||
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|
||||
|
||||
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|
||||
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|
||||
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|
||||
|
||||
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|
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|
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|
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|
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|
||||
=======================================================================
|
||||
|
||||
Creative Commons is not a party to its public
|
||||
licenses. Notwithstanding, Creative Commons may elect to apply one of
|
||||
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|
||||
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|
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|
||||
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|
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|
||||
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|
||||
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|
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|
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|
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|
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|
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|
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|
||||
Creative Commons may be contacted at creativecommons.org.
|
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Binary file not shown.
|
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|
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@ -171,7 +171,7 @@ First of all we get the end of the memory region with the:
|
||||
phys_addr_t end = base + memblock_cap_size(base, &size);
|
||||
```
|
||||
|
||||
`memblock_cap_size` adjusts `size` that `base + size` will not overflow. Its implementation is pretty easy:
|
||||
`memblock_cap_size` adjusts `size` so that `base + size` will not overflow. Its implementation is pretty easy:
|
||||
|
||||
```C
|
||||
static inline phys_addr_t memblock_cap_size(phys_addr_t base, phys_addr_t *size)
|
||||
@ -337,10 +337,10 @@ There is also `memblock_reserve` function which does the same as `memblock_add`,
|
||||
|
||||
Of course this is not the full API. Memblock provides APIs not only for adding `memory` and `reserved` memory regions, but also:
|
||||
|
||||
* memblock_remove - removes memory region from memblock;
|
||||
* memblock_find_in_range - finds free area in given range;
|
||||
* memblock_free - releases memory region in memblock;
|
||||
* for_each_mem_range - iterates through memblock areas.
|
||||
* `memblock_remove` - removes memory region from memblock;
|
||||
* `memblock_find_in_range` - finds free area in given range;
|
||||
* `memblock_free` - releases memory region in memblock;
|
||||
* `for_each_mem_range` - iterates through memblock areas.
|
||||
|
||||
and many more....
|
||||
|
||||
@ -349,8 +349,8 @@ Getting info about memory regions
|
||||
|
||||
Memblock also provides an API for getting information about allocated memory regions in the `memblock`. It is split in two parts:
|
||||
|
||||
* get_allocated_memblock_memory_regions_info - getting info about memory regions;
|
||||
* get_allocated_memblock_reserved_regions_info - getting info about reserved regions.
|
||||
* `get_allocated_memblock_memory_regions_info` - getting info about memory regions;
|
||||
* `get_allocated_memblock_reserved_regions_info` - getting info about reserved regions.
|
||||
|
||||
Implementation of these functions is easy. Let's look at `get_allocated_memblock_reserved_regions_info` for example:
|
||||
|
||||
@ -401,9 +401,9 @@ And you will see something like this:
|
||||
|
||||
Memblock also has support in [debugfs](http://en.wikipedia.org/wiki/Debugfs). If you run the kernel on another architecture than `X86` you can access:
|
||||
|
||||
* /sys/kernel/debug/memblock/memory
|
||||
* /sys/kernel/debug/memblock/reserved
|
||||
* /sys/kernel/debug/memblock/physmem
|
||||
* `/sys/kernel/debug/memblock/memory`
|
||||
* `/sys/kernel/debug/memblock/reserved`
|
||||
* `/sys/kernel/debug/memblock/physmem`
|
||||
|
||||
to get a dump of the `memblock` contents.
|
||||
|
||||
|
||||
@ -36,9 +36,9 @@ Base virtual address and size of the `fix-mapped` area are presented by the two
|
||||
#define FIXADDR_START (FIXADDR_TOP - FIXADDR_SIZE)
|
||||
```
|
||||
|
||||
Here `__end_of_permanent_fixed_addresses` is an element of the `fixed_addresses` enum and as I wrote above: Every fix-mapped address is represented by an integer index which is defined in the `fixed_addresses`. `PAGE_SHIFT` determines the size of a page. For example size of the one page we can get with the `1 << PAGE_SHIFT` expression.
|
||||
Here `__end_of_permanent_fixed_addresses` is an element of the `fixed_addresses` enum and as I wrote above, every fix-mapped address is represented by an integer index which is defined in the `fixed_addresses`. `PAGE_SHIFT` determines the size of a page. For example size of the one page we can get with the `1 << PAGE_SHIFT` expression.
|
||||
|
||||
In our case we need to get the size of the fix-mapped area, but not only of one page, that's why we are using `__end_of_permanent_fixed_addresses` for getting the size of the fix-mapped area. The `__end_of_permanent_fixed_addresses` is the last index of the `fixed_addresses` enum or in other words the `__end_of_permanent_fixed_addresses` contains amount of pages in a fixed-mapped area. So if multiply value of the `__end_of_permanent_fixed_addresses` on a page size value we will get size of fix-mapped area. In my case it's a little more than `536` kilobytes. In your case it might be a different number, because the size depends on amount of the fix-mapped addresses which are depends on your kernel's configuration.
|
||||
In our case we need to get the size of the fix-mapped area, but not only of one page, that's why we are using `__end_of_permanent_fixed_addresses` for getting the size of the fix-mapped area. The `__end_of_permanent_fixed_addresses` is the last index of the `fixed_addresses` enum or in other words the `__end_of_permanent_fixed_addresses` contains amount of pages in a fixed-mapped area. So if we multiply the value of the `__end_of_permanent_fixed_addresses` on a page size value we will get size of fix-mapped area. In my case it's a little more than `536` kilobytes. In your case it might be a different number, because the size depends on amount of the fix-mapped addresses which depends on your kernel configuration.
|
||||
|
||||
The second `FIXADDR_START` macro just subtracts the fix-mapped area size from the last address of the fix-mapped area to get its base virtual address. `FIXADDR_TOP` is a rounded up address from the base address of the [vsyscall](https://lwn.net/Articles/446528/) space:
|
||||
|
||||
@ -46,8 +46,8 @@ The second `FIXADDR_START` macro just subtracts the fix-mapped area size from th
|
||||
#define FIXADDR_TOP (round_up(VSYSCALL_ADDR + PAGE_SIZE, 1<<PMD_SHIFT) - PAGE_SIZE)
|
||||
```
|
||||
|
||||
The `fixed_addresses` enums are used as an index to get the virtual address by the `fix_to_virt` function. Implementation of this function is easy:
|
||||
|
||||
The `fixed_addresses` enums are used as indexes to get the virtual addresses by the `fix_to_virt` function. Implementation of this function is easy:
|
||||
|
||||
```C
|
||||
static __always_inline unsigned long fix_to_virt(const unsigned int idx)
|
||||
{
|
||||
@ -65,7 +65,7 @@ first of all it checks that the index given for the `fixed_addresses` enum is no
|
||||
Here we shift left the given index of a `fix-mapped` area on the `PAGE_SHIFT` which determines size of a page as I wrote above and subtract it from the `FIXADDR_TOP` which is the highest address of the `fix-mapped` area:
|
||||
|
||||
```
|
||||
+-----------------+
|
||||
+-----------------+
|
||||
| PAGE 1 | FIXADDR_TOP (virt address)
|
||||
| PAGE 2 |
|
||||
| PAGE 3 |
|
||||
@ -104,7 +104,7 @@ ioremap
|
||||
The Linux kernel provides many different primitives to manage memory. For this moment we will touch `I/O memory`. Every device is controlled by reading/writing from/to its registers. For example a driver can turn off/on a device by writing to its registers or get the state of a device by reading from its registers. Besides registers, many devices have buffers where a driver can write something or read from there. As we know for this moment there are two ways to access device's registers and data buffers:
|
||||
|
||||
* through the I/O ports;
|
||||
* mapping of the all registers to the memory address space;
|
||||
* mapping of all the registers to the memory address space;
|
||||
|
||||
In the first case every control register of a device has a number of input and output port. A device driver can read from a port and write to it with two `in` and `out` instructions which we already saw. If you want to know about currently registered port regions, you can learn about them by accessing `/proc/ioports`:
|
||||
|
||||
@ -208,7 +208,7 @@ Here `RTC_IO_EXTENT` is the size of the memory region and it is `0x8`, `"rtc"` i
|
||||
#define RTC_PORT(x) (0x70 + (x))
|
||||
```
|
||||
|
||||
So with the `request_region(RTC_PORT(0), size, "rtc")` we register a memory region that starts at `0x70` and and has a size of `0x8`. Let's look at `/proc/ioports`:
|
||||
So with the `request_region(RTC_PORT(0), size, "rtc")` we register a memory region that starts at `0x70` and has a size of `0x8`. Let's look at `/proc/ioports`:
|
||||
|
||||
```
|
||||
~$ sudo cat /proc/ioports | grep rtc
|
||||
@ -499,7 +499,7 @@ static inline void __native_flush_tlb_single(unsigned long addr)
|
||||
}
|
||||
```
|
||||
|
||||
or call `__flush_tlb` which just updates the `cr3` register as we have seen. After this step execution of the `__early_set_fixmap` function is finished and we can go back to the `__early_ioremap` implementation. When we have set up the fixmap area for the given address, we need to save the base virtual address of the I/O Re-mapped area in the `prev_map` using the `slot` index:
|
||||
or call `__flush_tlb` which just updates the `cr3` register as we have seen. After this step execution of the `__early_set_fixmap` function is finished and we can go back to the `__early_ioremap` implementation. When we have set up the fixmap area for the given address, we need to save the base virtual address of the I/O remapped area in the `prev_map` using the `slot` index:
|
||||
|
||||
```C
|
||||
prev_map[slot] = (void __iomem *)(offset + slot_virt[slot]);
|
||||
|
||||
@ -61,7 +61,7 @@ $ sudo cat /proc/ioports
|
||||
...
|
||||
```
|
||||
|
||||
can show us lists of currently registered port regions used for input or output communication with a device. All memory-mapped I/O addresses are not used by the kernel directly. So, before the Linux kernel can use such memory, it must map it to the virtual memory space which is the main purpose of the `ioremap` mechanism. Note that we saw only early `ioremap` in the previous [part](https://0xax.gitbook.io/linux-insides/summary/mm/linux-mm-2). Soon we will look at the implementation of the non-early `ioremap` function. But before this we must learn other things, like a different types of memory allocators and etc., because in other way it will be very difficult to understand it.
|
||||
can show us lists of currently registered port regions used for input or output communication with a device. All memory-mapped I/O addresses are not used by the kernel directly. So, before the Linux kernel can use such memory, it must map it to the virtual memory space which is the main purpose of the `ioremap` mechanism. Note that we saw only early `ioremap` in the previous [part](https://0xax.gitbook.io/linux-insides/summary/mm/linux-mm-2). Soon we will look at the implementation of the non-early `ioremap` function. But before this we must learn other things, like different types of memory allocators and etc., because otherwise it will be very difficult to understand it.
|
||||
|
||||
So, before we will move on to the non-early [memory management](https://en.wikipedia.org/wiki/Memory_management) of the Linux kernel, we will see some mechanisms which provide special abilities for [debugging](https://en.wikipedia.org/wiki/Debugging), check of [memory leaks](https://en.wikipedia.org/wiki/Memory_leak), memory control and etc. It will be easier to understand how memory management arranged in the Linux kernel after learning of all of these things.
|
||||
|
||||
@ -90,7 +90,7 @@ Here we allocate memory for the `A` structure and tries to print value of the `a
|
||||
gcc test.c -o test
|
||||
```
|
||||
|
||||
The [compiler](https://en.wikipedia.org/wiki/GNU_Compiler_Collection) will not show us warning that `a` filed is not uninitialized. But if we will run this program with [valgrind](https://en.wikipedia.org/wiki/Valgrind) tool, we will see the following output:
|
||||
The [compiler](https://en.wikipedia.org/wiki/GNU_Compiler_Collection) will not show us warning that `a` field is not uninitialized. But if we will run this program with [valgrind](https://en.wikipedia.org/wiki/Valgrind) tool, we will see the following output:
|
||||
|
||||
```
|
||||
~$ valgrind --leak-check=yes ./test
|
||||
@ -98,12 +98,12 @@ The [compiler](https://en.wikipedia.org/wiki/GNU_Compiler_Collection) will not s
|
||||
==28469== Copyright (C) 2002-2015, and GNU GPL'd, by Julian Seward et al.
|
||||
==28469== Using Valgrind-3.11.0 and LibVEX; rerun with -h for copyright info
|
||||
==28469== Command: ./test
|
||||
==28469==
|
||||
==28469==
|
||||
==28469== Conditional jump or move depends on uninitialised value(s)
|
||||
==28469== at 0x4E820EA: vfprintf (in /usr/lib64/libc-2.22.so)
|
||||
==28469== by 0x4E88D48: printf (in /usr/lib64/libc-2.22.so)
|
||||
==28469== by 0x4005B9: main (in /home/alex/test)
|
||||
==28469==
|
||||
==28469==
|
||||
==28469== Use of uninitialised value of size 8
|
||||
==28469== at 0x4E7E0BB: _itoa_word (in /usr/lib64/libc-2.22.so)
|
||||
==28469== by 0x4E8262F: vfprintf (in /usr/lib64/libc-2.22.so)
|
||||
@ -122,7 +122,7 @@ To enable this mechanism in the Linux kernel, you need to enable the `CONFIG_KME
|
||||
Kernel hacking
|
||||
-> Memory Debugging
|
||||
```
|
||||
|
||||
|
||||
menu of the Linux kernel configuration:
|
||||
|
||||
|
||||
@ -140,7 +140,7 @@ config X86
|
||||
...
|
||||
```
|
||||
|
||||
So, there is no anything which is specific for other architectures.
|
||||
So, there isn't anything which is specific for other architectures.
|
||||
|
||||
Ok, so we know that `kmemcheck` provides mechanism to check usage of `uninitialized memory` in the Linux kernel and how to enable it. How it does these checks? When the Linux kernel tries to allocate some memory i.e. something is called like this:
|
||||
|
||||
@ -148,7 +148,7 @@ Ok, so we know that `kmemcheck` provides mechanism to check usage of `uninitiali
|
||||
struct my_struct *my_struct = kmalloc(sizeof(struct my_struct), GFP_KERNEL);
|
||||
```
|
||||
|
||||
or in other words somebody wants to access a [page](https://en.wikipedia.org/wiki/Page_%28computer_memory%29), a [page fault](https://en.wikipedia.org/wiki/Page_fault) exception is generated. This is achieved by the fact that the `kmemcheck` marks memory pages as `non-present` (more about this you can read in the special part which is devoted to [Paging](https://0xax.gitbook.io/linux-insides/summary/theory/linux-theory-1)). If a `page fault` exception is occurred, the exception handler knows about it and in a case when the `kmemcheck` is enabled it transfers control to it. After the `kmemcheck` will finish its checks, the page will be marked as `present` and the interrupted code will be able to continue execution. There is little subtlety in this chain. When the first instruction of interrupted code will be executed, the `kmemcheck` will mark the page as `non-present` again. In this way next access to memory will be caught again.
|
||||
or in other words somebody wants to access a [page](https://en.wikipedia.org/wiki/Page_%28computer_memory%29), a [page fault](https://en.wikipedia.org/wiki/Page_fault) exception is generated. This is achieved by the fact that the `kmemcheck` marks memory pages as `non-present` (more about this you can read in the special part which is devoted to [Paging](https://0xax.gitbook.io/linux-insides/summary/theory/linux-theory-1)). If a `page fault` exception occurred, the exception handler knows about it and in a case when the `kmemcheck` is enabled it transfers control to it. After the `kmemcheck` will finish its checks, the page will be marked as `present` and the interrupted code will be able to continue execution. There is little subtlety in this chain. When the first instruction of interrupted code will be executed, the `kmemcheck` will mark the page as `non-present` again. In this way next access to memory will be caught again.
|
||||
|
||||
We just considered the `kmemcheck` mechanism from theoretical side. Now let's consider how it is implemented in the Linux kernel.
|
||||
|
||||
@ -215,9 +215,9 @@ if (!kmemcheck_selftest()) {
|
||||
printk(KERN_INFO "kmemcheck: Initialized\n");
|
||||
```
|
||||
|
||||
and return with the `EINVAL` if this check is failed. The `kmemcheck_selftest` function checks sizes of different memory access related [opcodes](https://en.wikipedia.org/wiki/Opcode) like `rep movsb`, `movzwq` and etc. If sizes of opcodes are equal to expected sizes, the `kmemcheck_selftest` will return `true` and `false` in other way.
|
||||
and return with the `EINVAL` if this check is failed. The `kmemcheck_selftest` function checks sizes of different memory access related [opcodes](https://en.wikipedia.org/wiki/Opcode) like `rep movsb`, `movzwq` and etc. If sizes of opcodes are equal to expected sizes, the `kmemcheck_selftest` will return `true` and `false` otherwise.
|
||||
|
||||
So when the somebody will call:
|
||||
So when somebody calls:
|
||||
|
||||
```C
|
||||
struct my_struct *my_struct = kmalloc(sizeof(struct my_struct), GFP_KERNEL);
|
||||
@ -236,7 +236,7 @@ if (kmemcheck_enabled && !(cachep->flags & SLAB_NOTRACK)) {
|
||||
}
|
||||
```
|
||||
|
||||
So, here we check that the if `kmemcheck` is enabled and the `SLAB_NOTRACK` bit is not set in flags we set `non-present` bit for the just allocated page. The `SLAB_NOTRACK` bit tell us to not track uninitialized memory. Additionally we check if a cache object has constructor (details will be considered in next parts) we mark allocated page as uninitialized or unallocated in other way. The `kmemcheck_alloc_shadow` function is defined in the [mm/kmemcheck.c](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/mm/kmemcheck.c) source code file and does following things:
|
||||
So, here we check that the if `kmemcheck` is enabled and the `SLAB_NOTRACK` bit is not set in flags we set `non-present` bit for the just allocated page. The `SLAB_NOTRACK` bit tell us to not track uninitialized memory. Additionally we check if a cache object has constructor (details will be considered in next parts) we mark allocated page as uninitialized or unallocated otherwise. The `kmemcheck_alloc_shadow` function is defined in the [mm/kmemcheck.c](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/mm/kmemcheck.c) source code file and does following things:
|
||||
|
||||
```C
|
||||
void kmemcheck_alloc_shadow(struct page *page, int order, gfp_t flags, int node)
|
||||
@ -276,7 +276,7 @@ void kmemcheck_hide_pages(struct page *p, unsigned int n)
|
||||
}
|
||||
```
|
||||
|
||||
Here we go through all pages and and tries to get `page table entry` for each page. If this operation was successful, we unset present bit and set hidden bit in each page. In the end we flush [translation lookaside buffer](https://en.wikipedia.org/wiki/Translation_lookaside_buffer), because some pages was changed. From this point allocated pages are tracked by the `kmemcheck`. Now, as `present` bit is unset, the [page fault](https://en.wikipedia.org/wiki/Page_fault) execution will be occurred right after the `kmalloc` will return pointer to allocated space and a code will try to access this memory.
|
||||
Here we go through all pages and try to get `page table entry` for each page. If this operation was successful, we unset present bit and set hidden bit in each page. In the end we flush [translation lookaside buffer](https://en.wikipedia.org/wiki/Translation_lookaside_buffer), because some pages was changed. From this point allocated pages are tracked by the `kmemcheck`. Now, as `present` bit is unset, the [page fault](https://en.wikipedia.org/wiki/Page_fault) execution will be occurred right after the `kmalloc` will return pointer to allocated space and a code will try to access this memory.
|
||||
|
||||
As you may remember from the [second part](https://0xax.gitbook.io/linux-insides/summary/initialization/linux-initialization-2) of the Linux kernel initialization chapter, the `page fault` handler is located in the [arch/x86/mm/fault.c](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/arch/x86/mm/fault.c) source code file and represented by the `do_page_fault` function. We can see following check from the beginning of the `do_page_fault` function:
|
||||
|
||||
@ -296,7 +296,7 @@ __do_page_fault(struct pt_regs *regs, unsigned long error_code,
|
||||
}
|
||||
```
|
||||
|
||||
The `kmemcheck_active` gets `kmemcheck_context` [per-cpu](https://0xax.gitbook.io/linux-insides/summary/concepts/linux-cpu-1) structure and return the result of comparison of the `balance` field of this structure with zero:
|
||||
The `kmemcheck_active` gets `kmemcheck_context` [per-cpu](https://0xax.gitbook.io/linux-insides/summary/concepts/linux-cpu-1) structure and returns the result of comparison of the `balance` field of this structure with zero:
|
||||
|
||||
```
|
||||
bool kmemcheck_active(struct pt_regs *regs)
|
||||
@ -314,7 +314,7 @@ if (kmemcheck_fault(regs, address, error_code))
|
||||
return;
|
||||
```
|
||||
|
||||
First of all the `kmemcheck_fault` function checks that the fault was occurred by the correct reason. At first we check the [flags register](https://en.wikipedia.org/wiki/FLAGS_register) and check that we are in normal kernel mode:
|
||||
First of all the `kmemcheck_fault` function checks that the fault occurred by the correct reason. At first we check the [flags register](https://en.wikipedia.org/wiki/FLAGS_register) and check that we are in normal kernel mode:
|
||||
|
||||
```C
|
||||
if (regs->flags & X86_VM_MASK)
|
||||
@ -323,7 +323,7 @@ if (regs->cs != __KERNEL_CS)
|
||||
return false;
|
||||
```
|
||||
|
||||
If these checks wasn't successful we return from the `kmemcheck_fault` function as it was not `kmemcheck` related page fault. After this we try to lookup a `page table entry` related to the faulted address and if we can't find it we return:
|
||||
If these checks weren't successful we return from the `kmemcheck_fault` function as it was not `kmemcheck` related page fault. After this we try to lookup a `page table entry` related to the faulted address and if we can't find it we return:
|
||||
|
||||
```C
|
||||
pte = kmemcheck_pte_lookup(address);
|
||||
@ -331,7 +331,7 @@ if (!pte)
|
||||
return false;
|
||||
```
|
||||
|
||||
Last two steps of the `kmemcheck_fault` function is to call the `kmemcheck_access` function which check access to the given page and show addresses again by setting present bit in the given page. The `kmemcheck_access` function does all main job. It check current instruction which caused a page fault. If it will find an error, the context of this error will be saved by `kmemcheck` to the ring queue:
|
||||
Last two steps of the `kmemcheck_fault` function is to call the `kmemcheck_access` function which check access to the given page and show addresses again by setting present bit in the given page. The `kmemcheck_access` function does all main job. It checks current instruction which caused a page fault. If it finds an error, the context of this error will be saved by `kmemcheck` to the ring queue:
|
||||
|
||||
```C
|
||||
static struct kmemcheck_error error_fifo[CONFIG_KMEMCHECK_QUEUE_SIZE];
|
||||
@ -343,7 +343,7 @@ The `kmemcheck` mechanism declares special [tasklet](https://0xax.gitbook.io/lin
|
||||
static DECLARE_TASKLET(kmemcheck_tasklet, &do_wakeup, 0);
|
||||
```
|
||||
|
||||
which runs the `do_wakeup` function from the [arch/x86/mm/kmemcheck/error.c](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/arch/x86/mm/kmemcheck/error.c) source code file when it will be scheduled to run.
|
||||
which runs the `do_wakeup` function from the [arch/x86/mm/kmemcheck/error.c](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/arch/x86/mm/kmemcheck/error.c) source code file when it is scheduled to run.
|
||||
|
||||
The `do_wakeup` function will call the `kmemcheck_error_recall` function which will print errors collected by `kmemcheck`. As we already saw the:
|
||||
|
||||
|
||||
25
Makefile
Normal file
25
Makefile
Normal file
@ -0,0 +1,25 @@
|
||||
### HELP
|
||||
|
||||
.PHONY: help
|
||||
help: ## Print help
|
||||
@egrep "(^### |^\S+:.*##\s)" Makefile | sed 's/^###\s*//' | sed 's/^\(\S*\)\:.*##\s*\(.*\)/ \1 - \2/'
|
||||
|
||||
### DOCKER
|
||||
|
||||
.PHONY: run
|
||||
run: image ## docker run ...
|
||||
(docker stop linux-insides-book 2>&1) > /dev/null || true
|
||||
docker run --detach --rm -p 4000:4000 --name linux-insides-book --hostname linux-insides-book linux-insides-book
|
||||
|
||||
.PHONY: image
|
||||
image: ## docker image build ...
|
||||
docker image build --rm --squash --label linux-insides --tag linux-insides-book:latest -f Dockerfile . 2> /dev/null || \
|
||||
docker image build --rm --label linux-insides --tag linux-insides-book:latest -f Dockerfile .
|
||||
|
||||
### LAUNCH BROWSER
|
||||
|
||||
.PHONY: browse
|
||||
browse: ## Launch broweser
|
||||
@timeout 60 sh -c 'until nc -z 127.0.0.1 4000; do sleep 1; done' || true
|
||||
@(uname | grep Darwin > /dev/null) && open http://127.0.0.1:4000 || true
|
||||
@(uname | grep Linux > /dev/null) && xdg-open http://127.0.0.1:4000 || true
|
||||
Binary file not shown.
|
Before Width: | Height: | Size: 886 KiB After Width: | Height: | Size: 767 KiB |
Binary file not shown.
|
Before Width: | Height: | Size: 31 KiB After Width: | Height: | Size: 20 KiB |
Binary file not shown.
|
Before Width: | Height: | Size: 74 KiB After Width: | Height: | Size: 65 KiB |
Binary file not shown.
|
Before Width: | Height: | Size: 121 KiB After Width: | Height: | Size: 107 KiB |
Binary file not shown.
|
Before Width: | Height: | Size: 838 KiB After Width: | Height: | Size: 726 KiB |
Binary file not shown.
|
Before Width: | Height: | Size: 774 KiB After Width: | Height: | Size: 671 KiB |
Binary file not shown.
|
Before Width: | Height: | Size: 88 KiB After Width: | Height: | Size: 76 KiB |
@ -224,7 +224,7 @@ $ make -j4
|
||||
|
||||
`busybox` is an executable file - `/bin/busybox` that contains a set of standard tools like [coreutils](https://en.wikipedia.org/wiki/GNU_Core_Utilities). In the `busybox` menu we need to enable: `Build BusyBox as a static binary (no shared libs)` option:
|
||||
|
||||
|
||||
|
||||
|
||||
We can find this menu in the:
|
||||
|
||||
|
||||
22
README.md
22
README.md
@ -63,28 +63,14 @@ In order to run your own copy of the book with gitbook within a local container:
|
||||
systemctl restart docker.service
|
||||
```
|
||||
|
||||
2. Build container image
|
||||
2. Run docker image
|
||||
```bash
|
||||
docker image build \
|
||||
--rm --squash \
|
||||
--label linux-insides \
|
||||
--tag linux-insides-book:latest \
|
||||
-f Dockerfile .
|
||||
make run
|
||||
```
|
||||
|
||||
3. Create and run book in local container
|
||||
```bash
|
||||
docker run \
|
||||
--detach \
|
||||
--rm \
|
||||
-p 4000:4000 \
|
||||
--name linux-insides-book \
|
||||
--hostname linux-insides-book \
|
||||
linux-insides-book
|
||||
```
|
||||
3. Open your local copy of linux insides book under this url
|
||||
http://localhost:4000 or run `make browse`
|
||||
|
||||
4. Open your local copy of linux insides book under this url
|
||||
http://localhost:4000
|
||||
|
||||
Contributions
|
||||
--------------
|
||||
|
||||
@ -81,7 +81,7 @@ The topic of this part is `queued spinlocks`. This approach may help to solve bo
|
||||
|
||||
The basic idea of the `MCS` lock is that a thread spins on a local variable and each processor in the system has its own copy of this variable (see the previous paragraph). In other words this concept is built on top of the [per-cpu](https://0xax.gitbook.io/linux-insides/summary/concepts/linux-cpu-1) variables concept in the Linux kernel.
|
||||
|
||||
When the first thread wants to acquire a lock, it registers itself in the `queue`. In other words it will be added to the special `queue` and will acquire lock, because it is free for now. When the second thread wants to acquire the same lock before the first thread release it, this thread adds its own copy of the lock variable into this `queue`. In this case the first thread will contain a `next` field which will point to the second thread. From this moment, the second thread will wait until the first thread release its lock and notify `next` thread about this event. The first thread will be deleted from the `queue` and the second thread will be owner of a lock.
|
||||
When the first thread wants to acquire a lock, it registers itself in the `queue`. In other words it will be added to the special `queue` and will acquire lock, because it is free for now. When the second thread wants to acquire the same lock before the first thread releases it, this thread adds its own copy of the lock variable into this `queue`. In this case the first thread will contain a `next` field which will point to the second thread. From this moment, the second thread will wait until the first thread releases its lock and notifies `next` thread about this event. The first thread will be deleted from the `queue` and the second thread will be owner of a lock.
|
||||
|
||||
Schematically we can represent it like:
|
||||
|
||||
@ -175,7 +175,7 @@ All of these macros expand to the call of functions from the same header file. A
|
||||
typedef struct qspinlock {
|
||||
union {
|
||||
atomic_t val;
|
||||
|
||||
|
||||
struct {
|
||||
u8 locked;
|
||||
u8 pending;
|
||||
@ -335,7 +335,7 @@ This array allows to make four attempts of a lock acquisition for the four event
|
||||
* software interrupt context;
|
||||
* non-maskable interrupt context.
|
||||
|
||||
Notice that we did not touch `queue` yet. We no need in it, because for two threads it just leads to unnecessary latency for memory access. In other case, the first thread may release it lock before this moment. In this case the `lock->val` will contain `_Q_LOCKED_VAL | _Q_PENDING_VAL` and we will start to build `queue`. We start to build `queue` by the getting the local copy of the `qnodes` array of the processor which executes thread and calculate `tail` which will indicate the tail of the `queue` and `idx` which represents an index of the `qnodes` array:
|
||||
Notice that we did not touch `queue` yet. We do not need it, because for two threads it just leads to unnecessary latency for memory access. In other case, the first thread may release it lock before this moment. In this case the `lock->val` will contain `_Q_LOCKED_VAL | _Q_PENDING_VAL` and we will start to build `queue`. We start to build `queue` by the getting the local copy of the `qnodes` array of the processor which executes thread and calculate `tail` which will indicate the tail of the `queue` and `idx` which represents an index of the `qnodes` array:
|
||||
|
||||
```C
|
||||
queue:
|
||||
@ -376,7 +376,7 @@ because we no need in it anymore as lock is acquired. If the `queued_spin_tryloc
|
||||
next = NULL;
|
||||
```
|
||||
|
||||
and retrieve previous tail. The next step is to check that `queue` is not empty. In this case we need to link previous entry with the new. While waitaing for the MCS lock, the next pointer may have been set by another lock waiter. We optimistically load the next pointer & prefetch the cacheline for writing to reduce latency in the upcoming MCS unlock operation:
|
||||
and retrieve previous tail. The next step is to check that `queue` is not empty. In this case we need to link previous entry with the new. While waiting for the MCS lock, the next pointer may have been set by another lock waiter. We optimistically load the next pointer & prefetch the cacheline for writing to reduce latency in the upcoming MCS unlock operation:
|
||||
|
||||
```C
|
||||
if (old & _Q_TAIL_MASK) {
|
||||
@ -384,7 +384,7 @@ and retrieve previous tail. The next step is to check that `queue` is not empty.
|
||||
WRITE_ONCE(prev->next, node);
|
||||
|
||||
arch_mcs_spin_lock_contended(&node->locked);
|
||||
|
||||
|
||||
next = READ_ONCE(node->next);
|
||||
if (next)
|
||||
prefetchw(next);
|
||||
@ -399,14 +399,14 @@ Yes, from this moment we are in the head of the `queue`. But before we are able
|
||||
val = atomic_cond_read_acquire(&lock->val, !(VAL & _Q_LOCKED_PENDING_MASK));
|
||||
```
|
||||
|
||||
After both threads will release a lock, the head of the `queue` will hold a lock. In the end we just need to update the tail of the `queue` and remove current head from it.
|
||||
After both threads will release a lock, the head of the `queue` will hold a lock. In the end we just need to update the tail of the `queue` and remove current head from it.
|
||||
|
||||
That's all.
|
||||
|
||||
Conclusion
|
||||
--------------------------------------------------------------------------------
|
||||
|
||||
This is the end of the second part of the [synchronization primitives](https://en.wikipedia.org/wiki/Synchronization_%28computer_science%29) chapter in the Linux kernel. In the previous [part](https://0xax.gitbook.io/linux-insides/summary/syncprim/linux-sync-1) we already met the first synchronization primitive `spinlock` provided by the Linux kernel which is implemented as `ticket spinlock`. In this part we saw another implementation of the `spinlock` mechanism - `queued spinlock`. In the next part we will continue to dive into synchronization primitives in the Linux kernel.
|
||||
This is the end of the second part of the [synchronization primitives](https://en.wikipedia.org/wiki/Synchronization_%28computer_science%29) chapter in the Linux kernel. In the previous [part](https://0xax.gitbook.io/linux-insides/summary/syncprim/linux-sync-1) we already met the first synchronization primitive `spinlock` provided by the Linux kernel which is implemented as `ticket spinlock`. In this part we saw another implementation of the `spinlock` mechanism - `queued spinlock`. In the next part we will continue to dive into synchronization primitives in the Linux kernel.
|
||||
|
||||
If you have questions or suggestions, feel free to ping me in twitter [0xAX](https://twitter.com/0xAX), drop me [email](mailto:anotherworldofworld@gmail.com) or just create [issue](https://github.com/0xAX/linux-insides/issues/new).
|
||||
|
||||
@ -417,13 +417,13 @@ Links
|
||||
|
||||
* [spinlock](https://en.wikipedia.org/wiki/Spinlock)
|
||||
* [interrupt](https://en.wikipedia.org/wiki/Interrupt)
|
||||
* [interrupt handler](https://en.wikipedia.org/wiki/Interrupt_handler)
|
||||
* [interrupt handler](https://en.wikipedia.org/wiki/Interrupt_handler)
|
||||
* [API](https://en.wikipedia.org/wiki/Application_programming_interface)
|
||||
* [Test and Set](https://en.wikipedia.org/wiki/Test-and-set)
|
||||
* [MCS](http://www.cs.rochester.edu/~scott/papers/1991_TOCS_synch.pdf)
|
||||
* [per-cpu variables](https://0xax.gitbook.io/linux-insides/summary/concepts/linux-cpu-1)
|
||||
* [atomic instruction](https://en.wikipedia.org/wiki/Linearizability)
|
||||
* [CMPXCHG instruction](http://x86.renejeschke.de/html/file_module_x86_id_41.html)
|
||||
* [CMPXCHG instruction](http://x86.renejeschke.de/html/file_module_x86_id_41.html)
|
||||
* [LOCK instruction](http://x86.renejeschke.de/html/file_module_x86_id_159.html)
|
||||
* [NOP instruction](https://en.wikipedia.org/wiki/NOP)
|
||||
* [PREFETCHW instruction](http://www.felixcloutier.com/x86/PREFETCHW.html)
|
||||
|
||||
@ -243,12 +243,12 @@ static inline int signal_pending_state(long state, struct task_struct *p)
|
||||
return 0;
|
||||
if (!signal_pending(p))
|
||||
return 0;
|
||||
|
||||
|
||||
return (state & TASK_INTERRUPTIBLE) || __fatal_signal_pending(p);
|
||||
}
|
||||
```
|
||||
|
||||
We check that the `state` [bitmask](https://en.wikipedia.org/wiki/Mask_%28computing%29) contains `TASK_INTERRUPTIBLE` or `TASK_WAKEKILL` bits and if the bitmask does not contain this bit we exit. At the next step we check that the given task has a pending signal and exit if there is no. In the end we just check `TASK_INTERRUPTIBLE` bit in the `state` bitmask again or the [SIGKILL](https://en.wikipedia.org/wiki/Unix_signal#SIGKILL) signal. So, if our task has a pending signal, we will jump at the `interrupted` label:
|
||||
We check that the `state` [bitmask](https://en.wikipedia.org/wiki/Mask_%28computing%29) contains `TASK_INTERRUPTIBLE` or `TASK_WAKEKILL` bits and if the bitmask does not contain this bit we exit. At the next step we check that the given task has a pending signal and exit if there is not. In the end we just check `TASK_INTERRUPTIBLE` bit in the `state` bitmask again or the [SIGKILL](https://en.wikipedia.org/wiki/Unix_signal#SIGKILL) signal. So, if our task has a pending signal, we will jump at the `interrupted` label:
|
||||
|
||||
```C
|
||||
interrupted:
|
||||
@ -306,7 +306,7 @@ void up(struct semaphore *sem)
|
||||
EXPORT_SYMBOL(up);
|
||||
```
|
||||
|
||||
It looks almost the same as the `down` function. There are only two differences here. First of all we increment a counter of a `semaphore` if the list of waiters is empty. In other way we call the `__up` function from the same source code file. If the list of waiters is not empty we need to allow the first task from the list to acquire a lock:
|
||||
It looks almost the same as the `down` function. There are only two differences here. First of all we increment a counter of a `semaphore` if the list of waiters is empty. In other way we call the `__up` function from the same source code file. If the list of waiters is not empty we need to allow the first task from the list to acquire a lock:
|
||||
|
||||
```C
|
||||
static noinline void __sched __up(struct semaphore *sem)
|
||||
@ -326,7 +326,7 @@ That's all.
|
||||
Conclusion
|
||||
--------------------------------------------------------------------------------
|
||||
|
||||
This is the end of the third part of the [synchronization primitives](https://en.wikipedia.org/wiki/Synchronization_%28computer_science%29) chapter in the Linux kernel. In the two previous parts we already met the first synchronization primitive `spinlock` provided by the Linux kernel which is implemented as `ticket spinlock` and used for a very short time locks. In this part we saw yet another synchronization primitive - [semaphore](https://en.wikipedia.org/wiki/Semaphore_%28programming%29) which is used for long time locks as it leads to [context switch](https://en.wikipedia.org/wiki/Context_switch). In the next part we will continue to dive into synchronization primitives in the Linux kernel and will see next synchronization primitive - [mutex](https://en.wikipedia.org/wiki/Mutual_exclusion).
|
||||
This is the end of the third part of the [synchronization primitives](https://en.wikipedia.org/wiki/Synchronization_%28computer_science%29) chapter in the Linux kernel. In the two previous parts we already met the first synchronization primitive `spinlock` provided by the Linux kernel which is implemented as `ticket spinlock` and used for a very short time locks. In this part we saw yet another synchronization primitive - [semaphore](https://en.wikipedia.org/wiki/Semaphore_%28programming%29) which is used for long time locks as it leads to [context switch](https://en.wikipedia.org/wiki/Context_switch). In the next part we will continue to dive into synchronization primitives in the Linux kernel and will see next synchronization primitive - [mutex](https://en.wikipedia.org/wiki/Mutual_exclusion).
|
||||
|
||||
If you have questions or suggestions, feel free to ping me in twitter [0xAX](https://twitter.com/0xAX), drop me [email](mailto:anotherworldofworld@gmail.com) or just create [issue](https://github.com/0xAX/linux-insides/issues/new).
|
||||
|
||||
|
||||
@ -23,7 +23,7 @@ struct semaphore {
|
||||
};
|
||||
```
|
||||
|
||||
structure which holds information about state of a [lock](https://en.wikipedia.org/wiki/Lock_%28computer_science%29) and list of a lock waiters. Depends on the value of the `count` field, a `semaphore` can provide access to a resource of more than one wishing of this resource. The [mutex](https://en.wikipedia.org/wiki/Mutual_exclusion) concept is very similar to a [semaphore](https://en.wikipedia.org/wiki/Semaphore_%28programming%29) concept. But it has some differences. The main difference between `semaphore` and `mutex` synchronization primitive is that `mutex` has more strict semantic. Unlike a `semaphore`, only one [process](https://en.wikipedia.org/wiki/Process_%28computing%29) may hold `mutex` at one time and only the `owner` of a `mutex` may release or unlock it. Additional difference in implementation of `lock` [API](https://en.wikipedia.org/wiki/Application_programming_interface). The `semaphore` synchronization primitive forces rescheduling of processes which are in waiters list. The implementation of `mutex` lock `API` allows to avoid this situation and as a result expensive [context switches](https://en.wikipedia.org/wiki/Context_switch).
|
||||
structure which holds information about state of a [lock](https://en.wikipedia.org/wiki/Lock_%28computer_science%29) and list of a lock waiters. Depending on the value of the `count` field, a `semaphore` can provide access to a resource to more than one processes wishing to access this resource. The [mutex](https://en.wikipedia.org/wiki/Mutual_exclusion) concept is very similar to a [semaphore](https://en.wikipedia.org/wiki/Semaphore_%28programming%29) concept. But it has some differences. The main difference between `semaphore` and `mutex` synchronization primitive is that `mutex` has more strict semantic. Unlike a `semaphore`, only one [process](https://en.wikipedia.org/wiki/Process_%28computing%29) may hold `mutex` at one time and only the `owner` of a `mutex` may release or unlock it. Additional difference in implementation of `lock` [API](https://en.wikipedia.org/wiki/Application_programming_interface). The `semaphore` synchronization primitive forces rescheduling of processes which are in waiters list. The implementation of `mutex` lock `API` allows to avoid this situation and has expensive [context switches](https://en.wikipedia.org/wiki/Context_switch).
|
||||
|
||||
The `mutex` synchronization primitive represented by the following:
|
||||
|
||||
@ -47,13 +47,13 @@ struct mutex {
|
||||
};
|
||||
```
|
||||
|
||||
structure in the Linux kernel. This structure is defined in the [include/linux/mutex.h](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/include/linux/mutex.h) header file and contains similar to the `semaphore` structure set of fields. The first field of the `mutex` structure is - `count`. Value of this field represents state of a `mutex`. In a case when the value of the `count` field is `1`, a `mutex` is in `unlocked` state. When the value of the `count` field is `zero`, a `mutex` is in the `locked` state. Additionally value of the `count` field may be `negative`. In this case a `mutex` is in the `locked` state and has possible waiters.
|
||||
structure in the Linux kernel. This structure is defined in the [include/linux/mutex.h](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/include/linux/mutex.h) header file and contains a set of fields similar to the `semaphore` structure. The first field of the `mutex` structure is - `count`. Value of this field represents state of a `mutex`. In a case when the value of the `count` field is `1`, a `mutex` is in `unlocked` state. When the value of the `count` field is `zero`, a `mutex` is in the `locked` state. Additionally value of the `count` field may be `negative`. In this case a `mutex` is in the `locked` state and has possible waiters.
|
||||
|
||||
The next two fields of the `mutex` structure - `wait_lock` and `wait_list` are [spinlock](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/include/linux/mutex.h) for the protection of a `wait queue` and list of waiters which represents this `wait queue` for a certain lock. As you may notice, the similarity of the `mutex` and `semaphore` structures ends. Remaining fields of the `mutex` structure, as we may see depends on different configuration options of the Linux kernel.
|
||||
|
||||
The first field - `owner` represents [process](https://en.wikipedia.org/wiki/Process_%28computing%29) which acquired a lock. As we may see, existence of this field in the `mutex` structure depends on the `CONFIG_DEBUG_MUTEXES` or `CONFIG_MUTEX_SPIN_ON_OWNER` kernel configuration options. Main point of this field and the next `osq` fields is support of `optimistic spinning` which we will see later. The last two fields - `magic` and `dep_map` are used only in [debugging](https://en.wikipedia.org/wiki/Debugging) mode. The `magic` field is to storing a `mutex` related information for debugging and the second field - `lockdep_map` is for [lock validator](https://www.kernel.org/doc/Documentation/locking/lockdep-design.txt) of the Linux kernel.
|
||||
|
||||
Now, after we have considered the `mutex` structure, we may consider how this synchronization primitive works in the Linux kernel. As you may guess, a process which wants to acquire a lock, must to decrease value of the `mutex->count` if possible. And if a process wants to release a lock, it must to increase the same value. That's true. But as you may also guess, it is not so simple in the Linux kernel.
|
||||
Now, after we have considered the `mutex` structure, we may consider how this synchronization primitive works in the Linux kernel. As you may guess, a process who wants to acquire a lock, must to decrease value of the `mutex->count` if possible. And if a process wants to release a lock, it must to increase the same value. That's true. But as you may also guess, it is not so simple in the Linux kernel.
|
||||
|
||||
Actually, when a process try to acquire a `mutex`, there three possible paths:
|
||||
|
||||
@ -63,7 +63,7 @@ Actually, when a process try to acquire a `mutex`, there three possible paths:
|
||||
|
||||
which may be taken, depending on the current state of the `mutex`. The first path or `fastpath` is the fastest as you may understand from its name. Everything is easy in this case. Nobody acquired a `mutex`, so the value of the `count` field of the `mutex` structure may be directly decremented. In a case of unlocking of a `mutex`, the algorithm is the same. A process just increments the value of the `count` field of the `mutex` structure. Of course, all of these operations must be [atomic](https://en.wikipedia.org/wiki/Linearizability).
|
||||
|
||||
Yes, this looks pretty easy. But what happens if a process wants to acquire a `mutex` which is already acquired by other process? In this case, the control will be transferred to the second path - `midpath`. The `midpath` or `optimistic spinning` tries to [spin](https://en.wikipedia.org/wiki/Spinlock) with already familiar for us [MCS lock](http://www.cs.rochester.edu/~scott/papers/1991_TOCS_synch.pdf) while the lock owner is running. This path will be executed only if there are no other processes ready to run that have higher priority. This path is called `optimistic` because the waiting task will not be sleep and rescheduled. This allows to avoid expensive [context switch](https://en.wikipedia.org/wiki/Context_switch).
|
||||
Yes, this looks pretty easy. But what happens if a process wants to acquire a `mutex` which is already acquired by other process? In this case, the control will be transferred to the second path - `midpath`. The `midpath` or `optimistic spinning` tries to [spin](https://en.wikipedia.org/wiki/Spinlock) with already familiar for us [MCS lock](http://www.cs.rochester.edu/~scott/papers/1991_TOCS_synch.pdf) while the lock owner is running. This path will be executed only if there are no other processes ready to run that have higher priority. This path is called `optimistic` because the waiting task will not sleep and be rescheduled. This allows to avoid expensive [context switch](https://en.wikipedia.org/wiki/Context_switch).
|
||||
|
||||
In the last case, when the `fastpath` and `midpath` may not be executed, the last path - `slowpath` will be executed. This path acts like a [semaphore](https://en.wikipedia.org/wiki/Semaphore_%28programming%29) lock. If the lock is unable to be acquired by a process, this process will be added to `wait queue` which is represented by the following:
|
||||
|
||||
@ -77,7 +77,7 @@ struct mutex_waiter {
|
||||
};
|
||||
```
|
||||
|
||||
structure from the [include/linux/mutex.h](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/include/linux/mutex.h) header file and will be sleep. Before we will consider [API](https://en.wikipedia.org/wiki/Application_programming_interface) which is provided by the Linux kernel for manipulation with `mutexes`, let's consider the `mutex_waiter` structure. If you have read the [previous part](https://0xax.gitbook.io/linux-insides/summary/syncprim/linux-sync-3) of this chapter, you may notice that the `mutex_waiter` structure is similar to the `semaphore_waiter` structure from the [kernel/locking/semaphore.c](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/kernel/locking/semaphore.c) source code file:
|
||||
structure from the [include/linux/mutex.h](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/include/linux/mutex.h) header file and will sleep. Before we will consider [API](https://en.wikipedia.org/wiki/Application_programming_interface) which is provided by the Linux kernel for manipulation of `mutexes`, let's consider the `mutex_waiter` structure. If you have read the [previous part](https://0xax.gitbook.io/linux-insides/summary/syncprim/linux-sync-3) of this chapter, you may notice that the `mutex_waiter` structure is similar to the `semaphore_waiter` structure from the [kernel/locking/semaphore.c](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/kernel/locking/semaphore.c) source code file:
|
||||
|
||||
```C
|
||||
struct semaphore_waiter {
|
||||
@ -87,16 +87,16 @@ struct semaphore_waiter {
|
||||
};
|
||||
```
|
||||
|
||||
It also contains `list` and `task` fields which are represent entry of the mutex wait queue. The one difference here that the `mutex_waiter` does not contains `up` field, but contains the `magic` field which depends on the `CONFIG_DEBUG_MUTEXES` kernel configuration option and used to store a `mutex` related information for debugging purpose.
|
||||
It also contains `list` and `task` fields which represent entry of the mutex wait queue. The one difference here that the `mutex_waiter` does not contains `up` field, but contains the `magic` field which depends on the `CONFIG_DEBUG_MUTEXES` kernel configuration option and used to store a `mutex` related information for debugging purpose.
|
||||
|
||||
Now we know what is it `mutex` and how it is represented the Linux kernel. In this case, we may go ahead and start to look at the [API](https://en.wikipedia.org/wiki/Application_programming_interface) which the Linux kernel provides for manipulation of `mutexes`.
|
||||
Now we know what is a `mutex` and how it is represented the Linux kernel. In this case, we may go ahead and start to look at the [API](https://en.wikipedia.org/wiki/Application_programming_interface) which the Linux kernel provides for manipulation of `mutexes`.
|
||||
|
||||
Mutex API
|
||||
--------------------------------------------------------------------------------
|
||||
|
||||
Ok, in the previous paragraph we knew what is it `mutex` synchronization primitive and saw the `mutex` structure which represents `mutex` in the Linux kernel. Now it's time to consider [API](https://en.wikipedia.org/wiki/Application_programming_interface) for manipulation of mutexes. Description of the `mutex` API is located in the [include/linux/mutex.h](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/include/linux/mutex.h) header file. As always, before we will consider how to acquire and release a `mutex`, we need to know how to initialize it.
|
||||
Ok, in the previous paragraph we knew what is a `mutex` synchronization primitive and saw the `mutex` structure which represents `mutex` in the Linux kernel. Now it's time to consider [API](https://en.wikipedia.org/wiki/Application_programming_interface) for manipulation of mutexes. Description of the `mutex` API is located in the [include/linux/mutex.h](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/include/linux/mutex.h) header file. As always, before we will consider how to acquire and release a `mutex`, we need to know how to initialize it.
|
||||
|
||||
There are two approaches to initialize a `mutex`. The first is to do it statically. For this purpose the Linux kernel provides following:
|
||||
There are two approaches to initializing a `mutex`. The first is to do it statically. For this purpose the Linux kernel provides following:
|
||||
|
||||
```C
|
||||
#define DEFINE_MUTEX(mutexname) \
|
||||
@ -114,9 +114,9 @@ macro. Let's consider implementation of this macro. As we may see, the `DEFINE_M
|
||||
}
|
||||
```
|
||||
|
||||
This macro is defined in the [same](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/include/linux/mutex.h) header file and as we may understand it initializes fields of the `mutex` structure the initial values. The `count` field get initialized with the `1` which represents `unlocked` state of a mutex. The `wait_lock` [spinlock](https://en.wikipedia.org/wiki/Spinlock) get initialized to the unlocked state and the last field `wait_list` to empty [doubly linked list](https://0xax.gitbook.io/linux-insides/summary/datastructures/linux-datastructures-1).
|
||||
This macro is defined in the [same](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/include/linux/mutex.h) header file and as we may understand it initializes fields of the `mutex` structure to their initial values. The `count` field get initialized with the `1` which represents `unlocked` state of a mutex. The `wait_lock` [spinlock](https://en.wikipedia.org/wiki/Spinlock) get initialized to the unlocked state and the last field `wait_list` to empty [doubly linked list](https://0xax.gitbook.io/linux-insides/summary/datastructures/linux-datastructures-1).
|
||||
|
||||
The second approach allows us to initialize a `mutex` dynamically. To do this we need to call the `__mutex_init` function from the [kernel/locking/mutex.c](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/kernel/locking/mutex.c) source code file. Actually, the `__mutex_init` function rarely called directly. Instead of the `__mutex_init`, the:
|
||||
The second approach allows us to initialize a `mutex` dynamically. To do this we need to call the `__mutex_init` function from the [kernel/locking/mutex.c](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/kernel/locking/mutex.c) source code file. Actually, the `__mutex_init` function is rarely called directly. Instead of the `__mutex_init`, the:
|
||||
|
||||
```C
|
||||
# define mutex_init(mutex) \
|
||||
@ -150,7 +150,7 @@ As we may see the `__mutex_init` function takes three arguments:
|
||||
* `name` - name of mutex for debugging purpose;
|
||||
* `key` - key for [lock validator](https://www.kernel.org/doc/Documentation/locking/lockdep-design.txt).
|
||||
|
||||
At the beginning of the `__mutex_init` function, we may see initialization of the `mutex` state. We set it to `unlocked` state with the `atomic_set` function which atomically set the give variable to the given value. After this we may see initialization of the `spinlock` to the unlocked state which will protect `wait queue` of the `mutex` and initialization of the `wait queue` of the `mutex`. After this we clear owner of the `lock` and initialize optimistic queue by the call of the `osq_lock_init` function from the [include/linux/osq_lock.h](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/include/linux/osq_lock.h) header file. This function just sets the tail of the optimistic queue to the unlocked state:
|
||||
At the beginning of the `__mutex_init` function, we may see initialization of the `mutex` state. We set it to `unlocked` state with the `atomic_set` function which atomically sets the variable to the given value. After this we may see initialization of the `spinlock` to the unlocked state which will protect `wait queue` of the `mutex` and initialization of the `wait queue` of the `mutex`. After this we clear owner of the `lock` and initialize optimistic queue by the call of the `osq_lock_init` function from the [include/linux/osq_lock.h](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/include/linux/osq_lock.h) header file. This function just sets the tail of the optimistic queue to the unlocked state:
|
||||
|
||||
```C
|
||||
static inline bool osq_is_locked(struct optimistic_spin_queue *lock)
|
||||
@ -161,7 +161,7 @@ static inline bool osq_is_locked(struct optimistic_spin_queue *lock)
|
||||
|
||||
In the end of the `__mutex_init` function we may see the call of the `debug_mutex_init` function, but as I already wrote in previous parts of this [chapter](https://0xax.gitbook.io/linux-insides/summary/syncprim), we will not consider debugging related stuff in this chapter.
|
||||
|
||||
After the `mutex` structure is initialized, we may go ahead and will look at the `lock` and `unlock` API of `mutex` synchronization primitive. Implementation of `mutex_lock` and `mutex_unlock` functions located in the [kernel/locking/mutex.c](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/kernel/locking/mutex.c) source code file. First of all let's start from the implementation of the `mutex_lock`. It looks:
|
||||
After the `mutex` structure is initialized, we may go ahead and will look at the `lock` and `unlock` API of `mutex` synchronization primitive. Implementation of `mutex_lock` and `mutex_unlock` functions is located in the [kernel/locking/mutex.c](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/kernel/locking/mutex.c) source code file. First of all let's start from the implementation of the `mutex_lock`. It looks:
|
||||
|
||||
```C
|
||||
void __sched mutex_lock(struct mutex *lock)
|
||||
@ -176,7 +176,7 @@ We may see the call of the `might_sleep` macro from the [include/linux/kernel.h]
|
||||
|
||||
After the `might_sleep` macro, we may see the call of the `__mutex_fastpath_lock` function. This function is architecture-specific and as we consider [x86_64](https://en.wikipedia.org/wiki/X86-64) architecture in this book, the implementation of the `__mutex_fastpath_lock` is located in the [arch/x86/include/asm/mutex_64.h](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/arch/x86/include/asm/mutex_64.h) header file. As we may understand from the name of the `__mutex_fastpath_lock` function, this function will try to acquire lock in a fast path or in other words this function will try to decrement the value of the `count` of the given mutex.
|
||||
|
||||
Implementation of the `__mutex_fastpath_lock` function consists from two parts. The first part is [inline assembly](https://0xax.gitbook.io/linux-insides/summary/theory/linux-theory-3) statement. Let's look at it:
|
||||
Implementation of the `__mutex_fastpath_lock` function consists of two parts. The first part is [inline assembly](https://0xax.gitbook.io/linux-insides/summary/theory/linux-theory-3) statement. Let's look at it:
|
||||
|
||||
```C
|
||||
asm_volatile_goto(LOCK_PREFIX " decl %0\n"
|
||||
@ -205,13 +205,13 @@ exit:
|
||||
return;
|
||||
```
|
||||
|
||||
For this moment he implementation of the `__mutex_fastpath_lock` function looks pretty easy. But the value of the `mutex->counter` may be negative after increment. In this case the:
|
||||
For this moment the implementation of the `__mutex_fastpath_lock` function looks pretty easy. But the value of the `mutex->counter` may be negative after decrement. In this case the:
|
||||
|
||||
```C
|
||||
fail_fn(v);
|
||||
```
|
||||
|
||||
will be called after our inline assembly statement. The `fail_fn` is the second parameter of the `__mutex_fastpath_lock` function and represents pointer to function which represents `midpath/slowpath` paths to acquire the given lock. In our case the `fail_fn` is the `__mutex_lock_slowpath` function. Before we will look at the implementation of the `__mutex_lock_slowpath` function, let's finish with the implementation of the `mutex_lock` function. In the simplest way, the lock will be acquired successfully by a process and the `__mutex_fastpath_lock` will be finished. In this case, we just call the
|
||||
will be called after our inline assembly statement. The `fail_fn` is the second parameter of the `__mutex_fastpath_lock` function and represents pointer to function which represents `midpath/slowpath` paths to acquire the given lock. In our case the `fail_fn` is the `__mutex_lock_slowpath` function. Before we look at the implementation of the `__mutex_lock_slowpath` function, let's finish with the implementation of the `mutex_lock` function. In the simplest way, the lock will be acquired successfully by a process and the `__mutex_fastpath_lock` will be finished. In this case, we just call the
|
||||
|
||||
```C
|
||||
mutex_set_owner(lock);
|
||||
@ -254,7 +254,7 @@ if (mutex_optimistic_spin(lock, ww_ctx, use_ww_ctx)) {
|
||||
}
|
||||
```
|
||||
|
||||
First of all the `mutex_optimistic_spin` function check that we don't need to reschedule or in other words there are no other tasks ready to run that have higher priority. If this check was successful we need to update `MCS` lock wait queue with the current spin. In this way only one spinner can complete for the mutex at one time:
|
||||
First of all, `mutex_optimistic_spin` checks that we don't need to reschedule or in other words there are no other tasks ready to run that have higher priority. If this check was successful we need to update `MCS` lock wait queue with the current spin. In this way only one spinner can complete for the mutex at one time:
|
||||
|
||||
```C
|
||||
osq_lock(&lock->osq)
|
||||
@ -279,7 +279,7 @@ while (true) {
|
||||
}
|
||||
```
|
||||
|
||||
and try to acquire a lock. First of all we try to take current owner and if the owner exists (it may not exists in a case when a process already released a mutex) and we wait for it in the `mutex_spin_on_owner` function before the owner will release a lock. If new task with higher priority have appeared during wait of the lock owner, we break the loop and go to sleep. In other case, the process already may release a lock, so we try to acquire a lock with the `mutex_try_to_acquired`. If this operation finished successfully, we set new owner for the given mutex, removes ourself from the `MCS` wait queue and exit from the `mutex_optimistic_spin` function. At this state a lock will be acquired by a process and we enable [preemption](https://en.wikipedia.org/wiki/Preemption_%28computing%29) and exit from the `__mutex_lock_common` function:
|
||||
and try to acquire a lock. First of all we try to take current owner and if the owner exists (it may not exist in a case when a process already released a mutex) and we wait for it in the `mutex_spin_on_owner` function before the owner will release a lock. If new task with higher priority have appeared during wait of the lock owner, we break the loop and go to sleep. In other case, the process already may release a lock, so we try to acquire a lock with the `mutex_try_to_acquired`. If this operation finished successfully, we set new owner for the given mutex, removes ourself from the `MCS` wait queue and exit from the `mutex_optimistic_spin` function. At this stage, a lock will be acquired by a process and we enable [preemption](https://en.wikipedia.org/wiki/Preemption_%28computing%29) and exit from the `__mutex_lock_common` function:
|
||||
|
||||
```C
|
||||
if (mutex_optimistic_spin(lock, ww_ctx, use_ww_ctx)) {
|
||||
@ -303,7 +303,7 @@ static bool mutex_optimistic_spin(struct mutex *lock,
|
||||
#endif
|
||||
```
|
||||
|
||||
In all of these cases, the `__mutex_lock_common` function will acct like a `semaphore`. We try to acquire a lock again because the owner of a lock might already release a lock before this time:
|
||||
In all of these cases, the `__mutex_lock_common` function will act like a `semaphore`. We try to acquire a lock again because the owner of a lock might already release a lock before this time:
|
||||
|
||||
```C
|
||||
if (!mutex_is_locked(lock) &&
|
||||
@ -324,7 +324,7 @@ In a successful case we update the owner of a lock, enable preemption and exit f
|
||||
skip_wait:
|
||||
mutex_set_owner(lock);
|
||||
preempt_enable();
|
||||
return 0;
|
||||
return 0;
|
||||
```
|
||||
|
||||
In this case a lock will be acquired. If can't acquire a lock for now, we enter into the following loop:
|
||||
@ -338,7 +338,7 @@ for (;;) {
|
||||
if (unlikely(signal_pending_state(state, task))) {
|
||||
ret = -EINTR;
|
||||
goto err;
|
||||
}
|
||||
}
|
||||
|
||||
__set_task_state(task, state);
|
||||
|
||||
@ -348,7 +348,7 @@ for (;;) {
|
||||
|
||||
where try to acquire a lock again and exit if this operation was successful. Yes, we try to acquire a lock again right after unsuccessful try before the loop. We need to do it to make sure that we get a wakeup once a lock will be unlocked. Besides this, it allows us to acquire a lock after sleep. In other case we check the current process for pending [signals](https://en.wikipedia.org/wiki/Unix_signal) and exit if the process was interrupted by a `signal` during wait for a lock acquisition. In the end of loop we didn't acquire a lock, so we set the task state for `TASK_UNINTERRUPTIBLE` and go to sleep with call of the `schedule_preempt_disabled` function.
|
||||
|
||||
That's all. We have considered all three possible paths through which a process may pass when it will want to acquire a lock. Now let's consider how `mutex_unlock` is implemented. When the `mutex_unlock` will be called by a process which wants to release a lock, the `__mutex_fastpath_unlock` will be called from the [arch/x86/include/asm/mutex_64.h](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/arch/x86/include/asm/mutex_64.h) header file:
|
||||
That's all. We have considered all three possible paths through which a process may pass when it will want to acquire a lock. Now let's consider how `mutex_unlock` is implemented. When the `mutex_unlock` is called by a process which wants to release a lock, the `__mutex_fastpath_unlock` will be called from the [arch/x86/include/asm/mutex_64.h](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/arch/x86/include/asm/mutex_64.h) header file:
|
||||
|
||||
```C
|
||||
void __sched mutex_unlock(struct mutex *lock)
|
||||
@ -385,12 +385,12 @@ __mutex_unlock_slowpath(atomic_t *lock_count)
|
||||
}
|
||||
```
|
||||
|
||||
In the `__mutex_unlock_common_slowpath` function we will get the first entry from the wait queue if the wait queue is not empty and wakeup related process:
|
||||
In the `__mutex_unlock_common_slowpath` function we will get the first entry from the wait queue if the wait queue is not empty and wake up related process:
|
||||
|
||||
```C
|
||||
if (!list_empty(&lock->wait_list)) {
|
||||
struct mutex_waiter *waiter =
|
||||
list_entry(lock->wait_list.next, struct mutex_waiter, list);
|
||||
list_entry(lock->wait_list.next, struct mutex_waiter, list);
|
||||
wake_up_process(waiter->task);
|
||||
}
|
||||
```
|
||||
@ -423,7 +423,7 @@ Links
|
||||
* [Spinlock](https://en.wikipedia.org/wiki/Spinlock)
|
||||
* [Semaphore](https://en.wikipedia.org/wiki/Semaphore_%28programming%29)
|
||||
* [Synchronization primitives](https://en.wikipedia.org/wiki/Synchronization_%28computer_science%29)
|
||||
* [API](https://en.wikipedia.org/wiki/Application_programming_interface)
|
||||
* [API](https://en.wikipedia.org/wiki/Application_programming_interface)
|
||||
* [Locking mechanism](https://en.wikipedia.org/wiki/Lock_%28computer_science%29)
|
||||
* [Context switches](https://en.wikipedia.org/wiki/Context_switch)
|
||||
* [lock validator](https://www.kernel.org/doc/Documentation/locking/lockdep-design.txt)
|
||||
|
||||
@ -13,7 +13,7 @@ So, let's start.
|
||||
Reader/Writer semaphore
|
||||
--------------------------------------------------------------------------------
|
||||
|
||||
Actually there are two types of operations may be performed on the data. We may read data and make changes in data. Two fundamental operations - `read` and `write`. Usually (but not always), `read` operation is performed more often than `write` operation. In this case, it would be logical to we may lock data in such way, that some processes may read locked data in one time, on condition that no one will not change the data. The [readers/writer lock](https://en.wikipedia.org/wiki/Readers%E2%80%93writer_lock) allows us to get this lock.
|
||||
Actually there are two types of operations may be performed on the data. We may read data and make changes in data. Two fundamental operations - `read` and `write`. Usually (but not always), `read` operation is performed more often than `write` operation. In this case, it would be logical to lock data in such way, that some processes may read locked data in one time, on condition that no one will not change the data. The [readers/writer lock](https://en.wikipedia.org/wiki/Readers%E2%80%93writer_lock) allows us to get this lock.
|
||||
|
||||
When a process which wants to write something into data, all other `writer` and `reader` processes will be blocked until the process which acquired a lock, will not release it. When a process reads data, other processes which want to read the same data too, will not be locked and will be able to do this. As you may guess, implementation of the `reader/writer semaphore` is based on the implementation of the `normal semaphore`. We already familiar with the [semaphore](https://en.wikipedia.org/wiki/Semaphore_%28programming%29) synchronization primitive from the third [part](https://0xax.gitbook.io/linux-insides/summary/syncprim/linux-sync-4) of this chapter. From the theoretical side everything looks pretty simple. Let's look how `reader/writer semaphore` is represented in the Linux kernel.
|
||||
|
||||
@ -81,7 +81,7 @@ Reader/Writer semaphore API
|
||||
|
||||
So, we know a little about `reader/writer semaphores` from theoretical side, let's look on its implementation in the Linux kernel. All `reader/writer semaphores` related [API](https://en.wikipedia.org/wiki/Application_programming_interface) is located in the [include/linux/rwsem.h](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/include/linux/rwsem.h) header file.
|
||||
|
||||
As always Before we will consider an [API](https://en.wikipedia.org/wiki/Application_programming_interface) of the `reader/writer semaphore` mechanism in the Linux kernel, we need to know how to initialize the `rw_semaphore` structure. As we already saw in previous parts of this [chapter](https://0xax.gitbook.io/linux-insides/summary/syncprim), all [synchronization primitives](https://en.wikipedia.org/wiki/Synchronization_%28computer_science%29) may be initialized in two ways:
|
||||
As always, before we consider an [API](https://en.wikipedia.org/wiki/Application_programming_interface) of the `reader/writer semaphore` mechanism in the Linux kernel, we need to know how to initialize the `rw_semaphore` structure. As we already saw in previous parts of this [chapter](https://0xax.gitbook.io/linux-insides/summary/syncprim), all [synchronization primitives](https://en.wikipedia.org/wiki/Synchronization_%28computer_science%29) may be initialized in two ways:
|
||||
|
||||
* `statically`;
|
||||
* `dynamically`.
|
||||
@ -103,7 +103,7 @@ As we may see, the `DECLARE_RWSEM` macro just expands to the definition of the `
|
||||
.wait_lock = __RAW_SPIN_LOCK_UNLOCKED(name.wait_lock) \
|
||||
__RWSEM_OPT_INIT(name) \
|
||||
__RWSEM_DEP_MAP_INIT(name)
|
||||
}
|
||||
}
|
||||
```
|
||||
|
||||
and expands to the initialization of fields of `rw_semaphore` structure. First of all we initialize `count` field of the `rw_semaphore` structure to the `unlocked` state with `RWSEM_UNLOCKED_VALUE` macro from the [arch/x86/include/asm/rwsem.h](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/arch/x86/include/asm/rwsem.h) architecture specific header file:
|
||||
@ -193,7 +193,7 @@ void __sched down_write(struct rw_semaphore *sem)
|
||||
}
|
||||
```
|
||||
|
||||
We already met the `might_sleep` macro in the [previous part](https://0xax.gitbook.io/linux-insides/summary/syncprim/linux-sync-4). In short words, Implementation of the `might_sleep` macro depends on the `CONFIG_DEBUG_ATOMIC_SLEEP` kernel configuration option and if this option is enabled, this macro just prints a stack trace if it was executed in [atomic](https://en.wikipedia.org/wiki/Linearizability) context. As this macro is mostly for debugging purpose we will skip it and will go ahead. Additionally we will skip the next macro from the `down_read` function - `rwsem_acquire` which is related to the [lock validator](https://www.kernel.org/doc/Documentation/locking/lockdep-design.txt) of the Linux kernel, because this is topic of other part.
|
||||
We already met the `might_sleep` macro in the [previous part](https://0xax.gitbook.io/linux-insides/summary/syncprim/linux-sync-4). In short, implementation of the `might_sleep` macro depends on the `CONFIG_DEBUG_ATOMIC_SLEEP` kernel configuration option and if this option is enabled, this macro just prints a stack trace if it was executed in [atomic](https://en.wikipedia.org/wiki/Linearizability) context. As this macro is mostly for debugging purpose we will skip it and will go ahead. Additionally we will skip the next macro from the `down_read` function - `rwsem_acquire` which is related to the [lock validator](https://www.kernel.org/doc/Documentation/locking/lockdep-design.txt) of the Linux kernel, because this is topic of other part.
|
||||
|
||||
The only two things that remained in the `down_write` function is the call of the `LOCK_CONTENDED` macro which is defined in the [include/linux/lockdep.h](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/include/linux/lockdep.h) header file and setting of owner of a lock with the `rwsem_set_owner` function which sets owner to currently running process:
|
||||
|
||||
@ -292,7 +292,7 @@ if (rwsem_optimistic_spin(sem))
|
||||
return sem;
|
||||
```
|
||||
|
||||
We will skip implementation of the `rwsem_optimistic_spin` function, as it is similar on the `mutex_optimistic_spin` function which we saw in the [previous part](https://0xax.gitbook.io/linux-insides/summary/syncprim/linux-sync-4). In short words we check existence other tasks ready to run that have higher priority in the `rwsem_optimistic_spin` function. If there are such tasks, the process will be added to the [MCS](http://www.cs.rochester.edu/~scott/papers/1991_TOCS_synch.pdf) `waitqueue` and start to spin in the loop until a lock will be able to be acquired. If `optimistic spinning` is disabled, a process will be added to the and marked as waiting for write:
|
||||
We will skip implementation of the `rwsem_optimistic_spin` function, as it is similar on the `mutex_optimistic_spin` function which we saw in the [previous part](https://0xax.gitbook.io/linux-insides/summary/syncprim/linux-sync-4). In short words we check existence other tasks ready to run that have higher priority in the `rwsem_optimistic_spin` function. If there are such tasks, the process will be added to the [MCS](http://www.cs.rochester.edu/~scott/papers/1991_TOCS_synch.pdf) `waitqueue` and start to spin in the loop until a lock will be able to be acquired. If `optimistic spinning` is disabled, a process will be added to the `wait_list` and marked as waiting for write:
|
||||
|
||||
```C
|
||||
waiter.task = current;
|
||||
@ -356,7 +356,7 @@ static inline void __down_read(struct rw_semaphore *sem)
|
||||
}
|
||||
```
|
||||
|
||||
which increments value of the given `rw_semaphore->count` and call the `call_rwsem_down_read_failed` if this value is negative. In other way we jump at the label `1:` and exit. After this `read` lock will be successfully acquired. Notice that we check a sign of the `count` value as it may be negative, because as you may remember most significant [word](https://en.wikipedia.org/wiki/Word_%28computer_architecture%29) of the `rw_semaphore->count` contains negated number of active writers.
|
||||
which increments value of the given `rw_semaphore->count` and calls the `call_rwsem_down_read_failed` if this value is negative. In other way we jump at the label `1:` and exit. After this `read` lock will be successfully acquired. Notice that we check a sign of the `count` value as it may be negative, because as you may remember most significant [word](https://en.wikipedia.org/wiki/Word_%28computer_architecture%29) of the `rw_semaphore->count` contains negated number of active writers.
|
||||
|
||||
Let's consider case when a process wants to acquire a lock for `read` operation, but it is already locked. In this case the `call_rwsem_down_read_failed` function from the [arch/x86/lib/rwsem.S](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/arch/x86/lib/rwsem.S) assembly file will be called. If you will look at the implementation of this function, you will notice that it does the same that `call_rwsem_down_read_failed` function does. Except it calls the `rwsem_down_read_failed` function instead of `rwsem_dow_write_failed`. Now let's consider implementation of the `rwsem_down_read_failed` function. It starts from the adding a process to the `wait queue` and updating of value of the `rw_semaphore->counter`:
|
||||
|
||||
|
||||
@ -4,20 +4,20 @@ Synchronization primitives in the Linux kernel. Part 6.
|
||||
Introduction
|
||||
--------------------------------------------------------------------------------
|
||||
|
||||
This is the sixth part of the chapter which describes [synchronization primitives](https://en.wikipedia.org/wiki/Synchronization_\(computer_science\)) in the Linux kernel and in the previous parts we finished to consider different [readers-writer lock](https://en.wikipedia.org/wiki/Readers%E2%80%93writer_lock) synchronization primitives. We will continue to learn synchronization primitives in this part and start to consider a similar synchronization primitive which can be used to avoid the `writer starvation` problem. The name of this synchronization primitive is - `seqlock` or `sequential locks`.
|
||||
This is the sixth part of the chapter which describes [synchronization primitives](https://en.wikipedia.org/wiki/Synchronization_(computer_science)) in the Linux kernel and in the previous parts we finished to consider different [readers-writer lock](https://en.wikipedia.org/wiki/Readers%E2%80%93writer_lock) synchronization primitives. We will continue to learn synchronization primitives in this part and start to consider a similar synchronization primitive which can be used to avoid the `writer starvation` problem. The name of this synchronization primitive is - `seqlock` or `sequential locks`.
|
||||
|
||||
We know from the previous [part](https://0xax.gitbook.io/linux-insides/summary/syncprim/linux-sync-5) that [readers-writer lock](https://en.wikipedia.org/wiki/Readers%E2%80%93writer_lock) is a special lock mechanism which allows concurrent access for read-only operations, but an exclusive lock is needed for writing or modifying data. As we may guess, it may lead to a problem which is called `writer starvation`. In other words, a writer process can't acquire a lock as long as at least one reader process which acquired a lock holds it. So, in the situation when contention is high, it will lead to situation when a writer process which wants to acquire a lock will wait for it for a long time.
|
||||
|
||||
The `seqlock` synchronization primitive can help solve this problem.
|
||||
|
||||
As in all previous parts of this [book](https://github.com/0xAX/linux-insides/blob/master/SUMMARY.md), we will try to consider this synchronization primitive from the theoretical side and only than we will consider [API](https://en.wikipedia.org/wiki/Application_programming_interface) provided by the Linux kernel to manipulate with `seqlocks`.
|
||||
As in all previous parts of this [book](https://github.com/0xAX/linux-insides/blob/master/SUMMARY.md), we will try to consider this synchronization primitive from the theoretical side and only than we will consider [API](https://en.wikipedia.org/wiki/Application_programming_interface) provided by the Linux kernel to manipulate the `seqlocks`.
|
||||
|
||||
So, let's start.
|
||||
|
||||
Sequential lock
|
||||
--------------------------------------------------------------------------------
|
||||
|
||||
So, what is a `seqlock` synchronization primitive and how does it work? Let's try to answer on these questions in this paragraph. Actually `sequential locks` were introduced in the Linux kernel 2.6.x. Main point of this synchronization primitive is to provide fast and lock-free access to shared resources. Since the heart of `sequential lock` synchronization primitive is [spinlock](https://0xax.gitbook.io/linux-insides/summary/syncprim/linux-sync-1) synchronization primitive, `sequential locks` work in situations where the protected resources are small and simple. Additionally write access must be rare and also should be fast.
|
||||
So, what is a `seqlock` synchronization primitive and how does it work? Let's try to answer these questions in this paragraph. Actually `sequential locks` were introduced in the Linux kernel 2.6.x. Main point of this synchronization primitive is to provide fast and lock-free access to shared resources. Since the heart of `sequential lock` synchronization primitive is [spinlock](https://0xax.gitbook.io/linux-insides/summary/syncprim/linux-sync-1) synchronization primitive, `sequential locks` work in situations where the protected resources are small and simple. Additionally write access must be rare and also should be fast.
|
||||
|
||||
Work of this synchronization primitive is based on the sequence of events counter. Actually a `sequential lock` allows free access to a resource for readers, but each reader must check existence of conflicts with a writer. This synchronization primitive introduces a special counter. The main algorithm of work of `sequential locks` is simple: Each writer which acquired a sequential lock increments this counter and additionally acquires a [spinlock](https://0xax.gitbook.io/linux-insides/summary/syncprim/linux-sync-1). When this writer finishes, it will release the acquired spinlock to give access to other writers and increment the counter of a sequential lock again.
|
||||
|
||||
@ -114,7 +114,7 @@ So we just initialize counter of the given sequential lock to zero and additiona
|
||||
#endif
|
||||
```
|
||||
|
||||
As I already wrote in previous parts of this [chapter](https://0xax.gitbook.io/linux-insides/summary/syncprim) we will not consider [debugging](https://en.wikipedia.org/wiki/Debugging) and [lock validator](https://www.kernel.org/doc/Documentation/locking/lockdep-design.txt) related stuff in this part. So for now we just skip the `SEQCOUNT_DEP_MAP_INIT` macro. The second field of the given `seqlock_t` is `lock` initialized with the `__SPIN_LOCK_UNLOCKED` macro which is defined in the [include/linux/spinlock_types.h](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/include/linux/spinlock_types.h) header file. We will not consider implementation of this macro here as it just initialize [rawspinlock](https://0xax.gitbook.io/linux-insides/summary/syncprim/linux-sync-1) with architecture-specific methods (More abot spinlocks you may read in first parts of this [chapter](https://0xax.gitbook.io/linux-insides/summary/syncprim)).
|
||||
As I already wrote in previous parts of this [chapter](https://0xax.gitbook.io/linux-insides/summary/syncprim) we will not consider [debugging](https://en.wikipedia.org/wiki/Debugging) and [lock validator](https://www.kernel.org/doc/Documentation/locking/lockdep-design.txt) related stuff in this part. So for now we just skip the `SEQCOUNT_DEP_MAP_INIT` macro. The second field of the given `seqlock_t` is `lock` initialized with the `__SPIN_LOCK_UNLOCKED` macro which is defined in the [include/linux/spinlock_types.h](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/include/linux/spinlock_types.h) header file. We will not consider implementation of this macro here as it just initializes [rawspinlock](https://0xax.gitbook.io/linux-insides/summary/syncprim/linux-sync-1) with architecture-specific methods (More about spinlocks you may read in first parts of this [chapter](https://0xax.gitbook.io/linux-insides/summary/syncprim)).
|
||||
|
||||
We have considered the first way to initialize a sequential lock. Let's consider second way to do the same, but do it dynamically. We can initialize a sequential lock with the `seqlock_init` macro which is defined in the same [include/linux/seqlock.h](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/include/linux/seqlock.h) header file.
|
||||
|
||||
@ -164,7 +164,7 @@ static inline void read_seqlock_excl(seqlock_t *sl)
|
||||
static inline void read_sequnlock_excl(seqlock_t *sl)
|
||||
```
|
||||
|
||||
and others. Before we move on to considering the implementation of this [API](https://en.wikipedia.org/wiki/Application_programming_interface), we must know that actually there are two types of readers. The first type of reader never blocks a writer process. In this case writer will not wait for readers. The second type of reader which can lock. In this case, the locking reader will block the writer as it will wait while reader will not release its lock.
|
||||
and others. Before we move on to considering the implementation of this [API](https://en.wikipedia.org/wiki/Application_programming_interface), we must know that there actually are two types of readers. The first type of reader never blocks a writer process. In this case writer will not wait for readers. The second type of reader which can lock. In this case, the locking reader will block the writer as it will wait while reader will not release its lock.
|
||||
|
||||
First of all let's consider the first type of readers. The `read_seqbegin` function begins a seq-read [critical section](https://en.wikipedia.org/wiki/Critical_section).
|
||||
|
||||
@ -281,7 +281,7 @@ static inline void raw_write_seqcount_begin(seqcount_t *s)
|
||||
}
|
||||
```
|
||||
|
||||
When a writer process will finish to modify data, the `write_sequnlock` function must be called to release a lock and give access to other writers or readers. Let's consider at the implementation of the `write_sequnlock` function. It looks pretty simple:
|
||||
When a writer process will finish to modify data, the `write_sequnlock` function must be called to release a lock and give access to other writers or readers. Let's consider the implementation of the `write_sequnlock` function. It looks pretty simple:
|
||||
|
||||
```C
|
||||
static inline void write_sequnlock(seqlock_t *sl)
|
||||
@ -321,7 +321,7 @@ static inline void write_sequnlock_irq(seqlock_t *sl)
|
||||
|
||||
As we may see, these functions differ only in the initialization of spinlock. They call `spin_lock_irq` and `spin_unlock_irq` instead of `spin_lock` and `spin_unlock`.
|
||||
|
||||
Or for example `write_seqlock_irqsave` and `write_sequnlock_irqrestore` functions which are the same but used `spin_lock_irqsave` and `spin_unlock_irqsave` macro to use in [IRQ](https://en.wikipedia.org/wiki/Interrupt_request_\(PC_architecture\)) handlers.
|
||||
Or for example `write_seqlock_irqsave` and `write_sequnlock_irqrestore` functions which are the same but used `spin_lock_irqsave` and `spin_unlock_irqsave` macro to use in [IRQ](https://en.wikipedia.org/wiki/Interrupt_request_(PC_architecture)) handlers.
|
||||
|
||||
That's all.
|
||||
|
||||
@ -338,7 +338,7 @@ Links
|
||||
--------------------------------------------------------------------------------
|
||||
|
||||
* [synchronization primitives](https://en.wikipedia.org/wiki/Synchronization_\(computer_science\))
|
||||
* [readers-writer lock](https://en.wikipedia.org/wiki/Readers%E2%80%93writer_lock)
|
||||
* [readers-writer lock](https://en.wikipedia.org/wiki/Readers%E2%80%93writer_lock)
|
||||
* [spinlock](https://0xax.gitbook.io/linux-insides/summary/syncprim/linux-sync-1)
|
||||
* [critical section](https://en.wikipedia.org/wiki/Critical_section)
|
||||
* [lock validator](https://www.kernel.org/doc/Documentation/locking/lockdep-design.txt)
|
||||
|
||||
Binary file not shown.
|
Before Width: | Height: | Size: 469 KiB After Width: | Height: | Size: 407 KiB |
@ -51,7 +51,7 @@ asmlinkage const sys_call_ptr_t sys_call_table[__NR_syscall_max+1] = {
|
||||
};
|
||||
```
|
||||
|
||||
As we can see, the `sys_call_table` is an array of `__NR_syscall_max + 1` size where the `__NR_syscall_max` macro represents the maximum number of system calls for the given [architecture](https://en.wikipedia.org/wiki/List_of_CPU_architectures). This book is about the [x86_64](https://en.wikipedia.org/wiki/X86-64) architecture, so for our case the `__NR_syscall_max` is `547` and this is the correct number at the time of writing (current Linux kernel version is `5.0.0-rc7`). We can see this macro in the header file generated by [Kbuild](https://www.kernel.org/doc/Documentation/kbuild/makefiles.txt) during kernel compilation - include/generated/asm-offsets.h`:
|
||||
As we can see, the `sys_call_table` is an array of `__NR_syscall_max + 1` size where the `__NR_syscall_max` macro represents the maximum number of system calls for the given [architecture](https://en.wikipedia.org/wiki/List_of_CPU_architectures). This book is about the [x86_64](https://en.wikipedia.org/wiki/X86-64) architecture, so for our case the `__NR_syscall_max` is `547` and this is the correct number at the time of writing (current Linux kernel version is `5.0.0-rc7`). We can see this macro in the header file generated by [Kbuild](https://www.kernel.org/doc/Documentation/kbuild/makefiles.txt) during kernel compilation - `include/generated/asm-offsets.h`:
|
||||
|
||||
```C
|
||||
#define __NR_syscall_max 547
|
||||
@ -126,7 +126,7 @@ SYSCALL invokes an OS system-call handler at privilege level 0.
|
||||
It does so by loading RIP from the IA32_LSTAR MSR
|
||||
```
|
||||
|
||||
it means that we need to put the system call entry in to the `IA32_LSTAR` [model specific register](https://en.wikipedia.org/wiki/Model-specific_register). This operation takes place during the Linux kernel initialization process. If you have read the fourth [part](https://0xax.gitbook.io/linux-insides/summary/interrupts/linux-interrupts-4) of the chapter that describes interrupts and interrupt handling in the Linux kernel, you know that the Linux kernel calls the `trap_init` function during the initialization process. This function is defined in the [arch/x86/kernel/setup.c](https://github.com/torvalds/linux/blob/master/arch/x86/kernel/setup.c) source code file and executes the initialization of the `non-early` exception handlers like divide error, [coprocessor](https://en.wikipedia.org/wiki/Coprocessor) error etc. Besides the initialization of the `non-early` exceptions handlers, this function calls the `cpu_init` function from the [arch/x86/kernel/cpu/common.c](https://github.com/torvalds/linux/blob/master/arch/x86/kernel/cpu/common.c) source code file which besides initialization of `per-cpu` state, calls the `syscall_init` function from the same source code file.
|
||||
It means that we need to put the system call entry in to the `IA32_LSTAR` [model specific register](https://en.wikipedia.org/wiki/Model-specific_register). This operation takes place during the Linux kernel initialization process. If you have read the fourth [part](https://0xax.gitbook.io/linux-insides/summary/interrupts/linux-interrupts-4) of the chapter that describes interrupts and interrupt handling in the Linux kernel, you know that the Linux kernel calls the `trap_init` function during the initialization process. This function is defined in the [arch/x86/kernel/setup.c](https://github.com/torvalds/linux/blob/master/arch/x86/kernel/setup.c) source code file and executes the initialization of the `non-early` exception handlers like divide error, [coprocessor](https://en.wikipedia.org/wiki/Coprocessor) error, etc. Besides the initialization of the `non-early` exceptions handlers, this function calls the `cpu_init` function from the [arch/x86/kernel/cpu/common.c](https://github.com/torvalds/linux/blob/master/arch/x86/kernel/cpu/common.c) source code file which besides initialization of `per-cpu` state, calls the `syscall_init` function from the same source code file.
|
||||
|
||||
This function performs the initialization of the system call entry point. Let's look on the implementation of this function. It does not take parameters and first of all it fills two model specific registers:
|
||||
|
||||
@ -135,7 +135,7 @@ wrmsrl(MSR_STAR, ((u64)__USER32_CS)<<48 | ((u64)__KERNEL_CS)<<32);
|
||||
wrmsrl(MSR_LSTAR, entry_SYSCALL_64);
|
||||
```
|
||||
|
||||
The first model specific register - `MSR_STAR` contains `63:48` bits of the user code segment. These bits will be loaded to the `CS` and `SS` segment registers for the `sysret` instruction which provides functionality to return from a system call to user code with the related privilege. Also the `MSR_STAR` contains `47:32` bits from the kernel code that will be used as the base selector for `CS` and `SS` segment registers when user space applications execute a system call. In the second line of code we fill the `MSR_LSTAR` register with the `entry_SYSCALL_64` symbol that represents system call entry. The `entry_SYSCALL_64` is defined in the [arch/x86/entry/entry_64.S](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/arch/x86/entry/entry_64.S) assembly file and contains code related to the preparation performed before a system call handler will be executed (I already wrote about these preparations, read above). We will not consider the `entry_SYSCALL_64` now, but will return to it later in this chapter.
|
||||
The first model specific register - `MSR_STAR` contains `63:48` bits of the user code segment. These bits will be loaded to the `CS` and `SS` segment registers for the `sysret` instruction which provides functionality to return from a system call to user code with the related privilege. Also the `MSR_STAR` contains `47:32` bits from the kernel code that will be used as the base selector for `CS` and `SS` segment registers when user space applications execute a system call. In the second line of code we fill the `MSR_LSTAR` register with the `entry_SYSCALL_64` symbol that represents system call entry. The `entry_SYSCALL_64` is defined in the [arch/x86/entry/entry_64.S](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/arch/x86/entry/entry_64.S) assembly file and contains code related to the preparation performed before a system call handler is executed (I already wrote about these preparations, read above). We will not consider the `entry_SYSCALL_64` now, but will return to it later in this chapter.
|
||||
|
||||
After we have set the entry point for system calls, we need to set the following model specific registers:
|
||||
|
||||
@ -193,10 +193,10 @@ wrmsrl(MSR_SYSCALL_MASK,
|
||||
|
||||
These flags will be cleared during syscall initialization. That's all, it is the end of the `syscall_init` function and it means that system call entry is ready to work. Now we can see what will occur when a user application executes the `syscall` instruction.
|
||||
|
||||
Preparation before system call handler will be called
|
||||
Preparation before system call handler is called
|
||||
--------------------------------------------------------------------------------
|
||||
|
||||
As I already wrote, before a system call or an interrupt handler will be called by the Linux kernel we need to do some preparations. The `idtentry` macro performs the preparations required before an exception handler will be executed, the `interrupt` macro performs the preparations required before an interrupt handler will be called and the `entry_SYSCALL_64` will do the preparations required before a system call handler will be executed.
|
||||
As I already wrote, before a system call or an interrupt handler is called by the Linux kernel we need to do some preparations. The `idtentry` macro performs the preparations required before an exception handler is executed, the `interrupt` macro performs the preparations required before an interrupt handler is called and the `entry_SYSCALL_64` will do the preparations required before a system call handler is executed.
|
||||
|
||||
The `entry_SYSCALL_64` is defined in the [arch/x86/entry/entry_64.S](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/arch/x86/entry/entry_64.S) assembly file and starts from the following macro:
|
||||
|
||||
|
||||
@ -36,7 +36,7 @@ static inline void map_vsyscall(void) {}
|
||||
#endif
|
||||
```
|
||||
|
||||
As we can read in the help text, the `CONFIG_X86_VSYSCALL_EMULATION` configuration option: `Enable vsyscall emulation`. Why emulate `vsyscall`? Actually, the `vsyscall` is a legacy [ABI](https://en.wikipedia.org/wiki/Application_binary_interface) due to security reasons. Virtual system calls have fixed addresses, meaning that `vsyscall` page is still at the same location every time and the location of this page is determined in the `map_vsyscall` function. Let's look on the implementation of this function:
|
||||
As we can read in the help text, the `CONFIG_X86_VSYSCALL_EMULATION` configuration option: `Enable vsyscall emulation`. Why emulate `vsyscall`? Actually, the `vsyscall` is a legacy [ABI](https://en.wikipedia.org/wiki/Application_binary_interface) due to security reasons. Virtual system calls have fixed addresses, meaning that `vsyscall` page is still at the same location every time and the location of this page is determined in the `map_vsyscall` function. Let's look on the implementation of this function:
|
||||
|
||||
```C
|
||||
void __init map_vsyscall(void)
|
||||
@ -223,7 +223,7 @@ do_ret:
|
||||
regs->ip = caller;
|
||||
regs->sp += 8;
|
||||
return true;
|
||||
```
|
||||
```
|
||||
|
||||
That's all. Now let's look on the modern concept - `vDSO`.
|
||||
|
||||
@ -263,10 +263,10 @@ static int __init init_vdso(void)
|
||||
|
||||
#ifdef CONFIG_X86_X32_ABI
|
||||
init_vdso_image(&vdso_image_x32);
|
||||
#endif
|
||||
#endif
|
||||
```
|
||||
|
||||
Both functions initialize the `vdso_image` structure. This structure is defined in the two generated source code files: the [arch/x86/entry/vdso/vdso-image-64.c](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/arch/x86/entry/vdso/vdso-image-64.c) and the [arch/x86/entry/vdso/vdso-image-32.c](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/arch/x86/entry/vdso/vdso-image-32.c). These source code files generated by the [vdso2c](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/arch/x86/entry/vdso/vdso2c.c) program from the different source code files, represent different approaches to call a system call like `int 0x80`, `sysenter` and etc. The full set of the images depends on the kernel configuration.
|
||||
Both functions initialize the `vdso_image` structure. This structure is defined in the two generated source code files: the [arch/x86/entry/vdso/vdso-image-64.c](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/arch/x86/entry/vdso/vdso-image-64.c) and the [arch/x86/entry/vdso/vdso-image-32.c](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/arch/x86/entry/vdso/vdso-image-32.c). These source code files generated by the [vdso2c](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/arch/x86/entry/vdso/vdso2c.c) program from the different source code files, represent different approaches to call a system call like `int 0x80`, `sysenter`, etc. The full set of the images depends on the kernel configuration.
|
||||
|
||||
For example for the `x86_64` Linux kernel it will contain `vdso_image_64`:
|
||||
|
||||
@ -296,7 +296,7 @@ If our kernel is configured for the `x86` architecture or for the `x86_64` and c
|
||||
#endif
|
||||
```
|
||||
|
||||
As we can understand from the name of the `vdso_image` structure, it represents image of the `vDSO` for the certain mode of the system call entry. This structure contains information about size in bytes of the `vDSO` area that always a multiple of `PAGE_SIZE` (`4096` bytes), pointer to the text mapping, start and end address of the `alternatives` (set of instructions with better alternatives for the certain type of the processor) and etc. For example `vdso_image_64` looks like this:
|
||||
As we can understand from the name of the `vdso_image` structure, it represents image of the `vDSO` for the certain mode of the system call entry. This structure contains information about size in bytes of the `vDSO` area that's always a multiple of `PAGE_SIZE` (`4096` bytes), pointer to the text mapping, start and end address of the `alternatives` (set of instructions with better alternatives for the certain type of the processor), etc. For example `vdso_image_64` looks like this:
|
||||
|
||||
```C
|
||||
const struct vdso_image vdso_image_64 = {
|
||||
|
||||
@ -98,9 +98,9 @@ struct user_arg_ptr envp = { .ptr.native = __envp };
|
||||
return do_execveat_common(AT_FDCWD, filename, argv, envp, 0);
|
||||
```
|
||||
|
||||
The `do_execveat_common` function does main work - it executes a new program. This function takes similar set of arguments, but as you can see it takes five arguments instead of three. The first argument is the file descriptor that represent directory with our application, in our case the `AT_FDCWD` means that the given pathname is interpreted relative to the current working directory of the calling process. The fifth argument is flags. In our case we passed `0` to the `do_execveat_common`. We will check in a next step, so will see it latter.
|
||||
The `do_execveat_common` function does main work - it executes a new program. This function takes similar set of arguments, but as you can see it takes five arguments instead of three. The first argument is the file descriptor that represent directory with our application, in our case the `AT_FDCWD` means that the given pathname is interpreted relative to the current working directory of the calling process. The fifth argument is flags. In our case we passed `0` to the `do_execveat_common`. We will check in a next step, so will see it later.
|
||||
|
||||
First of all the `do_execveat_common` function checks the `filename` pointer and returns if it is `NULL`. After this we check flags of the current process that limit of running processes is not exceed:
|
||||
First of all the `do_execveat_common` function checks the `filename` pointer and returns if it is `NULL`. After this we check flags of the current process that limit of running processes is not exceeded:
|
||||
|
||||
```C
|
||||
if (IS_ERR(filename))
|
||||
@ -201,7 +201,7 @@ if (retval)
|
||||
|
||||
The `bprm_mm_init` defined in the same source code file and as we can understand from the function's name, it makes initialization of the memory descriptor or in other words the `bprm_mm_init` function initializes `mm_struct` structure. This structure defined in the [include/linux/mm_types.h](https://github.com/torvalds/linux/blob/master/include/linux/mm_types.h) header file and represents address space of a process. We will not consider implementation of the `bprm_mm_init` function because we do not know many important stuff related to the Linux kernel memory manager, but we just need to know that this function initializes `mm_struct` and populate it with a temporary stack `vm_area_struct`.
|
||||
|
||||
After this we calculate the count of the command line arguments which are were passed to the our executable binary, the count of the environment variables and set it to the `bprm->argc` and `bprm->envc` respectively:
|
||||
After this we calculate the count of the command line arguments which were passed to our executable binary, the count of the environment variables and set it to the `bprm->argc` and `bprm->envc` respectively:
|
||||
|
||||
```C
|
||||
bprm->argc = count(argv, MAX_ARG_STRINGS);
|
||||
@ -274,7 +274,7 @@ and call the:
|
||||
search_binary_handler(bprm);
|
||||
```
|
||||
|
||||
function. This function goes through the list of handlers that contains different binary formats. Currently the Linux kernel supports following binary formats:
|
||||
function. This function goes through the list of handlers that contains different binary formats. Currently the Linux kernel supports the following binary formats:
|
||||
|
||||
* `binfmt_script` - support for interpreted scripts that are starts from the [#!](https://en.wikipedia.org/wiki/Shebang_%28Unix%29) line;
|
||||
* `binfmt_misc` - support different binary formats, according to runtime configuration of the Linux kernel;
|
||||
@ -385,9 +385,9 @@ start_thread_common(struct pt_regs *regs, unsigned long new_ip,
|
||||
}
|
||||
```
|
||||
|
||||
The `start_thread_common` function fills `fs` segment register with zero and `es` and `ds` with the value of the data segment register. After this we set new values to the [instruction pointer](https://en.wikipedia.org/wiki/Program_counter), `cs` segments etc. In the end of the `start_thread_common` function we can see the `force_iret` macro that force a system call return via `iret` instruction. Ok, we prepared new thread to run in userspace and now we can return from the `exec_binprm` and now we are in the `do_execveat_common` again. After the `exec_binprm` will finish its execution we release memory for structures that was allocated before and return.
|
||||
The `start_thread_common` function fills `fs` segment register with zero and `es` and `ds` with the value of the data segment register. After this we set new values to the [instruction pointer](https://en.wikipedia.org/wiki/Program_counter), `cs` segments etc. In the end of the `start_thread_common` function we can see the `force_iret` macro that forces a system call return via `iret` instruction. Ok, we prepared new thread to run in userspace and now we can return from the `exec_binprm` and now we are in the `do_execveat_common` again. After the `exec_binprm` will finish its execution we release memory for structures that was allocated before and return.
|
||||
|
||||
After we returned from the `execve` system call handler, execution of our program will be started. We can do it, because all context related information already configured for this purpose. As we saw the `execve` system call does not return control to a process, but code, data and other segments of the caller process are just overwritten of the program segments. The exit from our application will be implemented through the `exit` system call.
|
||||
After we returned from the `execve` system call handler, execution of our program will be started. We can do it, because all context related information is already configured for this purpose. As we saw the `execve` system call does not return control to a process, but code, data and other segments of the caller process are just overwritten of the program segments. The exit from our application will be implemented through the `exit` system call.
|
||||
|
||||
That's all. From this point our program will be executed.
|
||||
|
||||
|
||||
@ -4,11 +4,11 @@ How does the `open` system call work
|
||||
Introduction
|
||||
--------------------------------------------------------------------------------
|
||||
|
||||
This is the fifth part of the chapter that describes [system calls](https://en.wikipedia.org/wiki/System_call) mechanism in the Linux kernel. Previous parts of this chapter described this mechanism in general. Now I will try to describe implementation of different system calls in the Linux kernel. Previous parts from this chapter and parts from other chapters of the books describe mostly deep parts of the Linux kernel that are faintly visible or fully invisible from the userspace. But the Linux kernel code is not only about itself. The vast of the Linux kernel code provides ability to our code. Due to the Linux kernel our programs can read/write from/to files and don't know anything about sectors, tracks and other parts of a disk structures, we can send data over network and don't build encapsulated network packets by hand and etc.
|
||||
This is the fifth part of the chapter that describes [system calls](https://en.wikipedia.org/wiki/System_call) mechanism in the Linux kernel. Previous parts of this chapter described the mechanism of system calls in general. I will now try to describe the implementation of different system calls in the Linux kernel. Previous parts from this chapter and parts of other chapters of the book mostly described deep parts of the Linux kernel that are barely visible or invisible from userspace. However, the greatness of the Linux kernel is not its singular existence, but its ability to enable our code to perform various useful functions such as reading/writing from/to files without the knowledge of details such as sectors, tracks and other nitty gritties of the disk layout. For eg., the kernel allows programs to send data over networks without our having to encapsulate network packets by hand etc.
|
||||
|
||||
I don't know how about you, but it is interesting to me not only how an operating system works, but how do my software interacts with it. As you may know, our programs interacts with the kernel through the special mechanism which is called [system call](https://en.wikipedia.org/wiki/System_call). So, I've decided to write series of parts which will describe implementation and behavior of system calls which we are using every day like `read`, `write`, `open`, `close`, `dup` and etc.
|
||||
I don't know how about you, but the inner workings of the operating system both fascinate and excite my curiosity greatly. As you may know, our programs interact with the kernel through a special mechanism called [system call](https://en.wikipedia.org/wiki/System_call). I will hence attempt to describe the implementation and behavior of system calls such as `read`, `write`, `open`, `close`, `dup` etc. in a series of articles.
|
||||
|
||||
I have decided to start from the description of the [open](http://man7.org/linux/man-pages/man2/open.2.html) system call. if you have written at least one `C` program, you should know that before we are able to read/write or execute other manipulations with a file we need to open it with the `open` function:
|
||||
Let me start with the description of the simplest (and commonly used) [open](http://man7.org/linux/man-pages/man2/open.2.html) system call. if you have done any `C` programming at all, you should know that a file must be opened using the `open` system call before we are able to read/write to it.
|
||||
|
||||
```C
|
||||
#include <fcntl.h>
|
||||
@ -28,12 +28,12 @@ int main(int argc, char *argv) {
|
||||
printf("file successfully opened\n");
|
||||
}
|
||||
|
||||
close(fd);
|
||||
close(fd);
|
||||
return 0;
|
||||
}
|
||||
```
|
||||
|
||||
In this case, the open is the function from standard library, but not system call. The standard library will call related system call for us. The `open` call will return a [file descriptor](https://en.wikipedia.org/wiki/File_descriptor) which is just a unique number within our process which is associated with the opened file. Now as we opened a file and got file descriptor as result of `open` call, we may start to interact with this file. We can write into, read from it and etc. List of opened file by a process is available via [proc](https://en.wikipedia.org/wiki/Procfs) filesystem:
|
||||
In this case, `open` is a function from standard library, but not the system call. The standard library will call the related system call for us. The `open` call will return a [file descriptor](https://en.wikipedia.org/wiki/File_descriptor) which is just a unique number within our process which is associated with the opened file. Now as we opened a file and got file descriptor as result of `open` call, we may start to interact with this file. We can write into, read from it and etc. List of opened file by a process is available via [proc](https://en.wikipedia.org/wiki/Procfs) filesystem:
|
||||
|
||||
```
|
||||
$ sudo ls /proc/1/fd/
|
||||
@ -42,14 +42,14 @@ $ sudo ls /proc/1/fd/
|
||||
1 11 13 15 19 20 22 24 26 28 3 31 33 35 37 39 40 42 44 46 48 5 51 54 57 59 60 62 65 7 9
|
||||
```
|
||||
|
||||
I am not going to describe more details about the `open` routine from the userspace view in this post, but mostly from the kernel side. if you are not very familiar with, you can get more info in the [man page](http://man7.org/linux/man-pages/man2/open.2.html).
|
||||
I am not going to describe more details about the `open` routine from the userspace view in this post, but mostly from the kernel side. If you are not very familiar with, you can get more info in the [man page](http://man7.org/linux/man-pages/man2/open.2.html).
|
||||
|
||||
So let's start.
|
||||
|
||||
Definition of the open system call
|
||||
--------------------------------------------------------------------------------
|
||||
|
||||
If you have read the [fourth part](https://github.com/0xAX/linux-insides/blob/master/SysCall/linux-syscall-4.md) of the [linux-insides](https://github.com/0xAX/linux-insides/blob/master/SUMMARY.md) book, you should know that system calls are defined with the help of `SYSCALL_DEFINE` macro. So, the `open` system call is not exception.
|
||||
If you have read the [fourth part](https://github.com/0xAX/linux-insides/blob/master/SysCall/linux-syscall-4.md) of the [linux-insides](https://github.com/0xAX/linux-insides/blob/master/SUMMARY.md) book, you should know that system calls are defined with the help of `SYSCALL_DEFINE` macro. So, the `open` system call is no exception.
|
||||
|
||||
Definition of the `open` system call is located in the [fs/open.c](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/fs/open.c) source code file and looks pretty small for the first view:
|
||||
|
||||
@ -63,7 +63,7 @@ SYSCALL_DEFINE3(open, const char __user *, filename, int, flags, umode_t, mode)
|
||||
}
|
||||
```
|
||||
|
||||
As you may guess, the `do_sys_open` function from the [same](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/fs/open.c) source code file does the main job. But before this function will be called, let's consider the `if` clause from which the implementation of the `open` system call starts:
|
||||
As you may guess, the `do_sys_open` function from the [same](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/fs/open.c) source code file does the main job. But before this function is called, let's consider the `if` clause from which the implementation of the `open` system call starts:
|
||||
|
||||
```C
|
||||
if (force_o_largefile())
|
||||
@ -81,19 +81,19 @@ As we may read in the [GNU C Library Reference Manual](https://www.gnu.org/softw
|
||||
|
||||
> off_t
|
||||
>
|
||||
> This is a signed integer type used to represent file sizes.
|
||||
> This is a signed integer type used to represent file sizes.
|
||||
> In the GNU C Library, this type is no narrower than int.
|
||||
> If the source is compiled with _FILE_OFFSET_BITS == 64 this
|
||||
> If the source is compiled with _FILE_OFFSET_BITS == 64 this
|
||||
> type is transparently replaced by off64_t.
|
||||
|
||||
and
|
||||
|
||||
> off64_t
|
||||
>
|
||||
> This type is used similar to off_t. The difference is that
|
||||
> This type is used similar to off_t. The difference is that
|
||||
> even on 32 bit machines, where the off_t type would have 32 bits,
|
||||
> off64_t has 64 bits and so is able to address files up to 2^63 bytes
|
||||
> in length. When compiling with _FILE_OFFSET_BITS == 64 this type
|
||||
> in length. When compiling with _FILE_OFFSET_BITS == 64 this type
|
||||
> is available under the name off_t.
|
||||
|
||||
So it is not hard to guess that the `off_t`, `off64_t` and `O_LARGEFILE` are about a file size. In the case of the Linux kernel, the `O_LARGEFILE` is used to disallow opening large files on 32bit systems if the caller didn't specify `O_LARGEFILE` flag during opening of a file. On 64bit systems we force on this flag in open system call. And the `force_o_largefile` macro from the [include/linux/fcntl.h](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/include/linux/fcntl.h#L7) Linux kernel header file confirms this:
|
||||
@ -108,7 +108,7 @@ This macro may be architecture-specific as for example for [IA-64](https://en.wi
|
||||
|
||||
So, as we may see the `force_o_largefile` is just a macro which expands to the `true` value in our case of [x86_64](https://en.wikipedia.org/wiki/X86-64) architecture. As we are considering 64-bit architecture, the `force_o_largefile` will be expanded to `true` and the `O_LARGEFILE` flag will be added to the set of flags which were passed to the `open` system call.
|
||||
|
||||
Now as we considered meaning of the `O_LARGEFILE` flag and `force_o_largefile` macro, we can proceed to the consideration of the implementation of the `do_sys_open` function. As I wrote above, this function is defined in the [same](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/fs/open.c) source code file and looks:
|
||||
Now as we considered meaning of the `O_LARGEFILE` flag and `force_o_largefile` macro, we can proceed to the consideration of the implementation of the `do_sys_open` function. As I wrote above, this function is defined in the [same](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/fs/open.c) source code file and looks:
|
||||
|
||||
```C
|
||||
long do_sys_open(int dfd, const char __user *filename, int flags, umode_t mode)
|
||||
@ -204,7 +204,7 @@ else
|
||||
op->mode = 0;
|
||||
```
|
||||
|
||||
Here we reset permissions in `open_flags` instance if a opened file wasn't temporary and wasn't open for creation. This is because:
|
||||
Here we reset permissions in `open_flags` instance if an open file wasn't temporary and wasn't open for creation. This is because:
|
||||
|
||||
> if neither O_CREAT nor O_TMPFILE is specified, then mode is ignored.
|
||||
|
||||
@ -216,7 +216,7 @@ At the next step we check that a file is not tried to be opened via [fanotify](h
|
||||
flags &= ~FMODE_NONOTIFY & ~O_CLOEXEC;
|
||||
```
|
||||
|
||||
We do this to not leak a [file descriptor](https://en.wikipedia.org/wiki/File_descriptor). By default, the new file descriptor is set to remain open across an `execve` system call, but the `open` system call supports `O_CLOEXEC` flag that can be used to change this default behaviour. So we do this to prevent leaking of a file descriptor when one thread opens a file to set `O_CLOEXEC` flag and in the same time the second process does a [fork](https://en.wikipedia.org/wiki/Fork_\(system_call\)) + [execve](https://en.wikipedia.org/wiki/Exec_\(system_call\)) and as you may remember that child will have copies of the parent's set of open file descriptors.
|
||||
We do this to not leak a [file descriptor](https://en.wikipedia.org/wiki/File_descriptor). By default, the new file descriptor is set to remain open across an `execve` system call, but the `open` system call supports `O_CLOEXEC` flag that can be used to change this default behaviour. So we do this to prevent leaking of a file descriptor when one thread opens a file to set `O_CLOEXEC` flag and in the same time the second process does a [fork](https://en.wikipedia.org/wiki/Fork_(system_call)) + [execve](https://en.wikipedia.org/wiki/Exec_(system_call)) and as you may remember that child will have copies of the parent's set of open file descriptors.
|
||||
|
||||
At the next step we check that if our flags contains `O_SYNC` flag, we apply `O_DSYNC` flag too:
|
||||
|
||||
@ -256,7 +256,7 @@ So, in this case the file itself is not opened, but operations like `dup`, `fcnt
|
||||
op->open_flag = flags;
|
||||
```
|
||||
|
||||
Now we have filled `open_flag` field which represents flags that will control opening of a file and `mode` that will represent `umask` of a new file if we open file for creation. There are still to fill last flags in the our `open_flags` structure. The next is `op->acc_mode` which represents access mode to a opened file. We already filled the `acc_mode` local variable with the initial value at the beginning of the `build_open_flags` and now we check last two flags related to access mode:
|
||||
Now we have filled `open_flag` field which represents flags that will control opening of a file and `mode` that will represent `umask` of a new file if we open file for creation. There are still to fill last flags in our `open_flags` structure. The next is `op->acc_mode` which represents access mode to a opened file. We already filled the `acc_mode` local variable with the initial value at the beginning of the `build_open_flags` and now we check last two flags related to access mode:
|
||||
|
||||
```C
|
||||
if (flags & O_TRUNC)
|
||||
@ -364,9 +364,9 @@ if (unlikely(filp == ERR_PTR(-ESTALE)))
|
||||
|
||||
Note that it is called three times. Actually, the Linux kernel will open the file in [RCU](https://www.kernel.org/doc/Documentation/RCU/whatisRCU.txt) mode. This is the most efficient way to open a file. If this try will be failed, the kernel enters the normal mode. The third call is relatively rare, only in the [nfs](https://en.wikipedia.org/wiki/Network_File_System) file system is likely to be used. The `path_openat` function executes `path lookup` or in other words it tries to find a `dentry` (what the Linux kernel uses to keep track of the hierarchy of files in directories) corresponding to a path.
|
||||
|
||||
The `path_openat` function starts from the call of the `get_empty_flip()` function that allocates a new `file` structure with some additional checks like do we exceed amount of opened files in the system or not and etc. After we have got allocated new `file` structure we call the `do_tmpfile` or `do_o_path` functions in a case if we have passed `O_TMPFILE | O_CREATE` or `O_PATH` flags during call of the `open` system call. These both cases are quite specific, so let's consider quite usual case when we want to open already existed file and want to read/write from/to it.
|
||||
The `path_openat` function starts from the call of the `get_empty_flip()` function that allocates a new `file` structure with some additional checks like do we exceed amount of opened files in the system or not and etc. After we have got allocated new `file` structure we call the `do_tmpfile` or `do_o_path` functions in a case if we have passed `O_TMPFILE | O_CREATE` or `O_PATH` flags during call of the `open` system call. Both these cases are quite specific, so let's consider quite usual case when we want to open already existed file and want to read/write from/to it.
|
||||
|
||||
In this case the `path_init` function will be called. This function performs some preporatory work before actual path lookup. This includes search of start position of path traversal and its metadata like `inode` of the path, `dentry inode` and etc. This can be `root` directory - `/` or current directory as in our case, because we use `AT_CWD` as starting point (see call of the `do_sys_open` at the beginning of the post).
|
||||
In this case the `path_init` function will be called. This function performs some preparatory work before actual path lookup. This includes search of start position of path traversal and its metadata like `inode` of the path, `dentry inode` and etc. This can be `root` directory - `/` or current directory as in our case, because we use `AT_CWD` as starting point (see call of the `do_sys_open` at the beginning of the post).
|
||||
|
||||
The next step after the `path_init` is the [loop](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/fs/namei.c#L3457) which executes the `link_path_walk` and `do_last`. The first function executes name resolution or in other words this function starts process of walking along a given path. It handles everything step by step except the last component of a file path. This handling includes checking of a permissions and getting a file component. As a file component is gotten, it is passed to `walk_component` that updates current directory entry from the `dcache` or asks underlying filesystem. This repeats before all path's components will not be handled in such way. After the `link_path_walk` will be executed, the `do_last` function will populate a `file` structure based on the result of the `link_path_walk`. As we reached last component of the given file path the `vfs_open` function from the `do_last` will be called.
|
||||
|
||||
@ -389,7 +389,7 @@ Links
|
||||
* [file descriptor](https://en.wikipedia.org/wiki/File_descriptor)
|
||||
* [proc](https://en.wikipedia.org/wiki/Procfs)
|
||||
* [GNU C Library Reference Manual](https://www.gnu.org/software/libc/manual/html_mono/libc.html#File-Position-Primitive)
|
||||
* [IA-64](https://en.wikipedia.org/wiki/IA-64)
|
||||
* [IA-64](https://en.wikipedia.org/wiki/IA-64)
|
||||
* [x86_64](https://en.wikipedia.org/wiki/X86-64)
|
||||
* [opendir](http://man7.org/linux/man-pages/man3/opendir.3.html)
|
||||
* [fanotify](http://man7.org/linux/man-pages/man7/fanotify.7.html)
|
||||
|
||||
@ -3,7 +3,7 @@ Limits on resources in Linux
|
||||
|
||||
Each process in the system uses certain amount of different resources like files, CPU time, memory and so on.
|
||||
|
||||
Such resources are not infinite and each process and we should have an instrument to manage it. Sometimes it is useful to know current limits for a certain resource or to change it's value. In this post we will consider such instruments that allow us to get information about limits for a process and increase or decrease such limits.
|
||||
Such resources are not infinite and each process and we should have an instrument to manage it. Sometimes it is useful to know current limits for a certain resource or to change its value. In this post we will consider such instruments that allow us to get information about limits for a process and increase or decrease such limits.
|
||||
|
||||
We will start from userspace view and then we will look how it is implemented in the Linux kernel.
|
||||
|
||||
@ -175,7 +175,7 @@ if (new_rlim) {
|
||||
check that the given `soft` limit does not exceed `hard` limit and in a case when the given resource is the maximum number of a file descriptors that hard limit is not greater than `sysctl_nr_open` value. The value of the `sysctl_nr_open` can be found via [procfs](https://en.wikipedia.org/wiki/Procfs):
|
||||
|
||||
```
|
||||
~$ cat /proc/sys/fs/nr_open
|
||||
~$ cat /proc/sys/fs/nr_open
|
||||
1048576
|
||||
```
|
||||
|
||||
|
||||
Binary file not shown.
|
Before Width: | Height: | Size: 40 KiB After Width: | Height: | Size: 16 KiB |
@ -8,7 +8,7 @@ In the fifth [part](https://0xax.gitbook.io/linux-insides/summary/booting/linux-
|
||||
|
||||
Yeah, there will be many different things, but many many and once again many work with **memory**.
|
||||
|
||||
In my view, memory management is one of the most complex parts of the Linux kernel and in system programming in general. This is why we need to get acquainted with paging, before we proceed with the kernel initialization stuff.
|
||||
In my view, memory management is one of the most complex parts of the Linux kernel and system programming in general. This is why we need to get acquainted with paging, before we proceed with the kernel initialization stuff.
|
||||
|
||||
`Paging` is a mechanism that translates a linear memory address to a physical address. If you have read the previous parts of this book, you may remember that we saw segmentation in real mode when physical addresses are calculated by shifting a segment register by four and adding an offset. We also saw segmentation in protected mode, where we used the descriptor tables and base addresses from descriptors with offsets to calculate the physical addresses. Now we will see paging in 64-bit mode.
|
||||
|
||||
@ -27,7 +27,7 @@ There are three paging modes:
|
||||
* PAE paging;
|
||||
* IA-32e paging.
|
||||
|
||||
We will only explain the last mode here. To enable the `IA-32e paging` paging mode we need to do following things:
|
||||
We will only explain the last mode here. To enable the `IA-32e paging` paging mode we need to do the following things:
|
||||
|
||||
* set the `CR0.PG` bit;
|
||||
* set the `CR4.PAE` bit;
|
||||
@ -211,7 +211,7 @@ Usually kernel's `.text` starts here with the `CONFIG_PHYSICAL_START` offset. We
|
||||
|
||||
```
|
||||
readelf -s vmlinux | grep ffffffff81000000
|
||||
1: ffffffff81000000 0 SECTION LOCAL DEFAULT 1
|
||||
1: ffffffff81000000 0 SECTION LOCAL DEFAULT 1
|
||||
65099: ffffffff81000000 0 NOTYPE GLOBAL DEFAULT 1 _text
|
||||
90766: ffffffff81000000 0 NOTYPE GLOBAL DEFAULT 1 startup_64
|
||||
```
|
||||
|
||||
@ -13,7 +13,7 @@ Now let's have a closer look on these components.
|
||||
|
||||
**ELF header**
|
||||
|
||||
The ELF header is located at the beginning of the object file. Its main purpose is to locate all other parts of the object file. The File header contains the following fields:
|
||||
The ELF header is located at the beginning of the object file. Its main purpose is to locate all other parts of the object file. The file header contains the following fields:
|
||||
|
||||
* ELF identification - array of bytes which helps identify the file as an ELF object file and also provides information about general object file characteristic;
|
||||
* Object file type - identifies the object file type. This field can describe that ELF file is a relocatable object file, an executable file, etc...;
|
||||
@ -109,12 +109,12 @@ The ELF object file also contains other fields/structures which you can find in
|
||||
vmlinux
|
||||
--------------------------------------------------------------------------------
|
||||
|
||||
`vmlinux` is also a relocatable ELF object file . We can take a look at it with the `readelf` util. First of all let's look at the header:
|
||||
`vmlinux` is also a relocatable ELF object file . We can take a look at it with the `readelf` utility. First of all let's look at the header:
|
||||
|
||||
```
|
||||
$ readelf -h vmlinux
|
||||
ELF Header:
|
||||
Magic: 7f 45 4c 46 02 01 01 00 00 00 00 00 00 00 00 00
|
||||
Magic: 7f 45 4c 46 02 01 01 00 00 00 00 00 00 00 00 00
|
||||
Class: ELF64
|
||||
Data: 2's complement, little endian
|
||||
Version: 1 (current)
|
||||
@ -147,7 +147,7 @@ We can then look this address up in the `vmlinux` ELF object with:
|
||||
|
||||
```
|
||||
$ readelf -s vmlinux | grep ffffffff81000000
|
||||
1: ffffffff81000000 0 SECTION LOCAL DEFAULT 1
|
||||
1: ffffffff81000000 0 SECTION LOCAL DEFAULT 1
|
||||
65099: ffffffff81000000 0 NOTYPE GLOBAL DEFAULT 1 _text
|
||||
90766: ffffffff81000000 0 NOTYPE GLOBAL DEFAULT 1 startup_64
|
||||
```
|
||||
@ -205,9 +205,9 @@ Program Headers:
|
||||
Segment Sections...
|
||||
00 .text .notes __ex_table .rodata __bug_table .pci_fixup .builtin_fw
|
||||
.tracedata __ksymtab __ksymtab_gpl __kcrctab __kcrctab_gpl
|
||||
__ksymtab_strings __param __modver
|
||||
01 .data .vvar
|
||||
02 .data..percpu
|
||||
__ksymtab_strings __param __modver
|
||||
01 .data .vvar
|
||||
02 .data..percpu
|
||||
03 .init.text .init.data .x86_cpu_dev.init .altinstructions
|
||||
.altinstr_replacement .iommu_table .apicdrivers .exit.text
|
||||
.smp_locks .data_nosave .bss .brk
|
||||
|
||||
@ -57,7 +57,7 @@ __asm__ [volatile] [goto] (AssemblerTemplate
|
||||
[ : GotoLabels ]);
|
||||
```
|
||||
|
||||
All parameters which are marked with squared brackets are optional. You may notice that if we skip the optional parameters and the modifiers `volatile` and `goto` we obtain the `basic` form.
|
||||
All parameters which are marked with squared brackets are optional. You may notice that if we skip the optional parameters and the modifiers `volatile` and `goto` we obtain the `basic` form.
|
||||
|
||||
Let's start to consider this in order. The first optional `qualifier` is `volatile`. This specifier tells the compiler that an assembly statement may produce `side effects`. In this case we need to prevent compiler optimizations related to the given assembly statement. In simple terms the `volatile` specifier instructs the compiler not to modify the statement and place it exactly where it was in the original code. As an example let's look at the following function from the [Linux kernel](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/arch/x86/include/asm/desc.h):
|
||||
|
||||
@ -157,7 +157,7 @@ These are input operands - variables `a` and `b`. We already know what the `r` q
|
||||
|
||||
First of all our values `5` and `10` will be put at the stack and then these values will be moved to the two general purpose registers: `%rdx` and `%rax`.
|
||||
|
||||
This way the `%rax` register is used for storing the value of the `b` as well as storing the result of the calculation. **NOTE** that I've used `gcc 6.3.1` version, so the resulted code of your compiler may differ.
|
||||
This way the `%rax` register is used for storing the value of the `b` as well as storing the result of the calculation. **NOTE** that I've used `gcc 6.3.1` version, so the resulted code of your compiler may differ.
|
||||
|
||||
We have looked at input and output parameters of an inline assembly statement. Before we move on to other constraints supported by `gcc`, there is one remaining part of the inline assembly statement we have not discussed yet - `clobbers`.
|
||||
|
||||
@ -231,7 +231,7 @@ int main(void)
|
||||
{
|
||||
unsigned long a[3] = {10000000000, 0, 1};
|
||||
unsigned long b = 5;
|
||||
|
||||
|
||||
__asm__ volatile("incq %0" :: "m" (a[0]));
|
||||
|
||||
printf("a[0] - b = %lu\n", a[0] - b);
|
||||
@ -288,12 +288,12 @@ Now the result is correct. If we look at the assembly output again:
|
||||
```assembly
|
||||
00000000004004f6 <main>:
|
||||
400404: 48 b8 00 e4 0b 54 02 movabs $0x2540be400,%rax
|
||||
40040b: 00 00 00
|
||||
40040b: 00 00 00
|
||||
40040e: 48 89 04 24 mov %rax,(%rsp)
|
||||
400412: 48 c7 44 24 08 00 00 movq $0x0,0x8(%rsp)
|
||||
400419: 00 00
|
||||
400419: 00 00
|
||||
40041b: 48 c7 44 24 10 01 00 movq $0x1,0x10(%rsp)
|
||||
400422: 00 00
|
||||
400422: 00 00
|
||||
400424: 48 ff 04 24 incq (%rsp)
|
||||
400428: 48 8b 04 24 mov (%rsp),%rax
|
||||
400431: 48 8d 70 fb lea -0x5(%rax),%rsi
|
||||
@ -306,14 +306,14 @@ we will see one difference here which is in the last two lines:
|
||||
400431: 48 8d 70 fb lea -0x5(%rax),%rsi
|
||||
```
|
||||
|
||||
Instead of constant folding, `GCC` now preserves calculations in the assembly and places the value of `a[0]` in the `%rax` register afterwards. In the end it just subtracts the constant value of `b` from the `%rax` register and puts result to the `%rsi`.
|
||||
Instead of constant folding, `GCC` now preserves calculations in the assembly and places the value of `a[0]` in the `%rax` register afterwards. In the end it just subtracts the constant value of `b` from the `%rax` register and puts the result to the `%rsi`.
|
||||
|
||||
Besides the `memory` specifier, we also see a new constraint here - `m`. This constraint tells the compiler to use the address of `a[0]`, instead of its value. So, now we are finished with `clobbers` and we may continue by looking at other constraints supported by `GCC` besides `r` and `m` which we have already seen.
|
||||
|
||||
Constraints
|
||||
---------------------------------------------------------------------------------
|
||||
|
||||
Now that we are finished with all three parts of an inline assembly statement, let's return to constraints. We already saw some constraints in the previous parts, like `r` which represents a `register` operand, `m` which represents a memory operand and `0-9` which represent an reused, indexed operand. Besides these `GCC` provides support for other constraints. For example the `i` constraint represents an `immediate` integer operand with know value:
|
||||
Now that we are finished with all three parts of an inline assembly statement, let's return to constraints. We already saw some constraints in the previous parts, like `r` which represents a `register` operand, `m` which represents a memory operand and `0-9` which represent a reused, indexed operand. Besides these `GCC` provides support for other constraints. For example the `i` constraint represents an `immediate` integer operand with known value:
|
||||
|
||||
```C
|
||||
#include <stdio.h>
|
||||
@ -388,7 +388,7 @@ int main(void)
|
||||
{
|
||||
static unsigned long arr[3] = {0, 1, 2};
|
||||
static unsigned long element;
|
||||
|
||||
|
||||
__asm__ volatile("movq 16+%1, %0" : "=r"(element) : "o"(arr));
|
||||
printf("%lu\n", element);
|
||||
return 0;
|
||||
@ -434,7 +434,7 @@ That's about all of the commonly used constraints in inline assembly statements.
|
||||
Architecture specific constraints
|
||||
--------------------------------------------------------------------------------
|
||||
|
||||
Before we finish, let's look at the set of special constraints. These constrains are architecture specific and as this book is specific to the [x86_64](https://en.wikipedia.org/wiki/X86-64) architecture, we will look at constraints related to it. First of all the set of `a` ... `d` and also `S` and `D` constraints represent [generic purpose](https://en.wikipedia.org/wiki/Processor_register) registers. In this case the `a` constraint corresponds to `%al`, `%ax`, `%eax` or `%rax` register depending on instruction size. The `S` and `D` constraints are `%si` and `%di` registers respectively. For example let's take our previous example. We can see in its assembly output that value of the `a` variable is stored in the `%eax` register. Now let's look at the assembly output of the same assembly, but with other constraint:
|
||||
Before we finish, let's look at the set of special constraints. These constrains are architecture specific and as this book is specific to the [x86_64](https://en.wikipedia.org/wiki/X86-64) architecture, we will look at constraints related to it. First of all the set of `a` ... `d` and also `S` and `D` constraints represent [generic purpose](https://en.wikipedia.org/wiki/Processor_register) registers. In this case the `a` constraint corresponds to `%al`, `%ax`, `%eax` or `%rax` register depending on instruction size. The `S` and `D` constraints are `%si` and `%di` registers respectively. For example let's take our previous example. We can see in its assembly output that value of the `a` variable is stored in the `%eax` register. Now let's look at the assembly output of the same assembly, but with other constraint:
|
||||
|
||||
```C
|
||||
#include <stdio.h>
|
||||
@ -464,7 +464,7 @@ Links
|
||||
--------------------------------------------------------------------------------
|
||||
|
||||
* [Linux kernel source code](https://github.com/torvalds/linux)
|
||||
* [assembly programming language](https://en.wikipedia.org/wiki/Assembly_language)
|
||||
* [assembly programming language](https://en.wikipedia.org/wiki/Assembly_language)
|
||||
* [GCC](https://en.wikipedia.org/wiki/GNU_Compiler_Collection)
|
||||
* [GNU extension](https://gcc.gnu.org/onlinedocs/gcc/C-Extensions.html)
|
||||
* [Global Descriptor Table](https://en.wikipedia.org/wiki/Global_Descriptor_Table)
|
||||
|
||||
Binary file not shown.
|
Before Width: | Height: | Size: 444 KiB After Width: | Height: | Size: 383 KiB |
Binary file not shown.
|
Before Width: | Height: | Size: 808 KiB After Width: | Height: | Size: 700 KiB |
@ -371,7 +371,7 @@ u64 get_jiffies_64(void)
|
||||
EXPORT_SYMBOL(get_jiffies_64);
|
||||
```
|
||||
|
||||
Note that the `get_jiffies_64` function does not implemented as `jiffies_read` for example:
|
||||
Note that the `get_jiffies_64` function is not implemented as `jiffies_read` for example:
|
||||
|
||||
```C
|
||||
static cycle_t jiffies_read(struct clocksource *cs)
|
||||
|
||||
@ -79,15 +79,15 @@ As you can see, the `jiffies` variable is very widely used in the Linux kernel [
|
||||
Introduction to `clocksource`
|
||||
--------------------------------------------------------------------------------
|
||||
|
||||
The `clocksource` concept represents the generic API for clock sources management in the Linux kernel. Why do we need a separate framework for this? Let's go back to the beginning. The `time` concept is the fundamental concept in the Linux kernel and other operating system kernels. And the timekeeping is one of the necessities to use this concept. For example Linux kernel must know and update the time elapsed since system startup, it must determine how long the current process has been running for every processor and many many more. Where the Linux kernel can get information about time? First of all it is Real Time Clock or [RTC](https://en.wikipedia.org/wiki/Real-time_clock) that represents by the a nonvolatile device. You can find a set of architecture-independent real time clock drivers in the Linux kernel in the [drivers/rtc](https://github.com/torvalds/linux/tree/master/drivers/rtc) directory. Besides this, each architecture can provide a driver for the architecture-dependent real time clock, for example - `CMOS/RTC` - [arch/x86/kernel/rtc.c](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/arch/x86/kernel/rtc.c) for the [x86](https://en.wikipedia.org/wiki/X86) architecture. The second is system timer - timer that excites [interrupts](https://en.wikipedia.org/wiki/Interrupt) with a periodic rate. For example, for [IBM PC](https://en.wikipedia.org/wiki/IBM_Personal_Computer) compatibles it was - [programmable interval timer](https://en.wikipedia.org/wiki/Programmable_interval_timer).
|
||||
The `clocksource` concept represents the generic API for clock sources management in the Linux kernel. Why do we need a separate framework for this? Let's go back to the beginning. The `time` concept is the fundamental concept in the Linux kernel and other operating system kernels. And the timekeeping is one of the necessities to use this concept. For example Linux kernel must know and update the time elapsed since system startup, it must determine how long the current process has been running for every processor and many many more. Where the Linux kernel can get information about time? First of all it is Real Time Clock or [RTC](https://en.wikipedia.org/wiki/Real-time_clock) that represents the nonvolatile device. You can find a set of architecture-independent real time clock drivers in the Linux kernel in the [drivers/rtc](https://github.com/torvalds/linux/tree/master/drivers/rtc) directory. Besides this, each architecture can provide a driver for the architecture-dependent real time clock, for example - `CMOS/RTC` - [arch/x86/kernel/rtc.c](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/arch/x86/kernel/rtc.c) for the [x86](https://en.wikipedia.org/wiki/X86) architecture. The second is system timer - timer that excites [interrupts](https://en.wikipedia.org/wiki/Interrupt) with a periodic rate. For example, for [IBM PC](https://en.wikipedia.org/wiki/IBM_Personal_Computer) compatibles it was - [programmable interval timer](https://en.wikipedia.org/wiki/Programmable_interval_timer).
|
||||
|
||||
We already know that for timekeeping purposes we can use `jiffies` in the Linux kernel. The `jiffies` can be considered as read only global variable which is updated with `HZ` frequency. We know that the `HZ` is a compile-time kernel parameter whose reasonable range is from `100` to `1000` [Hz](https://en.wikipedia.org/wiki/Hertz). So, it is guaranteed to have an interface for time measurement with `1` - `10` milliseconds resolution. Besides standard `jiffies`, we saw the `refined_jiffies` clock source in the previous part that is based on the `i8253/i8254` [programmable interval timer](https://en.wikipedia.org/wiki/Programmable_interval_timer) tick rate which is almost `1193182` hertz. So we can get something about `1` microsecond resolution with the `refined_jiffies`. In this time, [nanoseconds](https://en.wikipedia.org/wiki/Nanosecond) are the favorite choice for the time value units of the given `clocksource`.
|
||||
|
||||
The availability of more precise techniques for time intervals measurement is hardware-dependent. We just knew a little about `x86` dependent timers hardware. But each architecture provides own timers hardware. Earlier each architecture had own implementation for this purpose. Solution of this problem is an abstraction layer and associated API in a common code framework for managing various clock sources and independent of the timer interrupt. This common code framework became - `clocksource` framework.
|
||||
The availability of more precise techniques for time intervals measurement is hardware-dependent. We just knew a little about `x86` dependent timers hardware. But each architecture provides its own timer(s) hardware. Earlier each architecture had own implementation for this purpose. Solution of this problem is an abstraction layer and associated API in a common code framework for managing various clock sources and independent of the timer interrupt. This common code framework became - `clocksource` framework.
|
||||
|
||||
Generic timeofday and `clocksource` management framework moved a lot of timekeeping code into the architecture independent portion of the code, with the architecture-dependent portion reduced to defining and managing low-level hardware pieces of clocksources. It takes a large amount of funds to measure the time interval on different architectures with different hardware, and it is very complex. Implementation of the each clock related service is strongly associated with an individual hardware device and as you can understand, it results in similar implementations for different architectures.
|
||||
|
||||
Within this framework, each clock source is required to maintain a representation of time as a monotonically increasing value. As we can see in the Linux kernel code, nanoseconds are the favorite choice for the time value units of a clock source in this time. One of the main point of the clock source framework is to allow a user to select clock source among a range of available hardware devices supporting clock functions when configuring the system and selecting, accessing and scaling different clock sources.
|
||||
Within this framework, each clock source is required to maintain a representation of time as a monotonically increasing value. As we can see in the Linux kernel code, nanoseconds are the favorite choice for the time value units of a clock source at this time. One of the main point of the clock source framework is to allow a user to select clock source among a range of available hardware devices supporting clock functions when configuring the system and selecting, accessing and scaling different clock sources.
|
||||
|
||||
The `clocksource` structure
|
||||
--------------------------------------------------------------------------------
|
||||
@ -123,7 +123,7 @@ struct clocksource {
|
||||
} ____cacheline_aligned;
|
||||
```
|
||||
|
||||
We already saw the first field of the `clocksource` structure in the previous part - it is pointer to the `read` function that returns best counter selected by the clocksource framework. For example we use `jiffies_read` function to read `jiffies` value:
|
||||
We already saw the first field of the `clocksource` structure in the previous part - it is a pointer to the `read` function that returns best counter selected by the clocksource framework. For example we use `jiffies_read` function to read `jiffies` value:
|
||||
|
||||
```C
|
||||
static struct clocksource clocksource_jiffies = {
|
||||
@ -171,7 +171,7 @@ is not `100%` accurate. Instead the number is taken as close as possible to a na
|
||||
* `suspend` - suspend function for the clocksource;
|
||||
* `resume` - resume function for the clocksource;
|
||||
|
||||
The next field is the `max_cycles` and as we can understand from its name, this field represents maximum cycle value before potential overflow. And the last field is `owner` represents reference to a kernel [module](https://en.wikipedia.org/wiki/Loadable_kernel_module) that is owner of a clocksource. This is all. We just went through all the standard fields of the `clocksource` structure. But you can noted that we missed some fields of the `clocksource` structure. We can divide all of missed field on two types: Fields of the first type are already known for us. For example, they are `name` field that represents name of a `clocksource`, the `rating` field that helps to the Linux kernel to select the best clocksource and etc. The second type, fields which are dependent from the different Linux kernel configuration options. Let's look on these fields.
|
||||
The next field is the `max_cycles` and as we can understand from its name, this field represents maximum cycle value before potential overflow. And the last field is `owner` represents reference to a kernel [module](https://en.wikipedia.org/wiki/Loadable_kernel_module) that is owner of a clocksource. This is all. We just went through all the standard fields of the `clocksource` structure. But you might have noted that we missed some fields of the `clocksource` structure. We can divide all of missed field on two types: Fields of the first type are already known for us. For example, they are `name` field that represents name of a `clocksource`, the `rating` field that helps to the Linux kernel to select the best clocksource and etc. The second type, fields which are dependent from the different Linux kernel configuration options. Let's look on these fields.
|
||||
|
||||
The first field is the `archdata`. This field has `arch_clocksource_data` type and depends on the `CONFIG_ARCH_CLOCKSOURCE_DATA` kernel configuration option. This field is actual only for the [x86](https://en.wikipedia.org/wiki/X86) and [IA64](https://en.wikipedia.org/wiki/IA-64) architectures for this moment. And again, as we can understand from the field's name, it represents architecture-specific data for a clock source. For example, it represents `vDSO` clock mode:
|
||||
|
||||
@ -180,7 +180,7 @@ struct arch_clocksource_data {
|
||||
int vclock_mode;
|
||||
};
|
||||
```
|
||||
|
||||
|
||||
for the `x86` architectures. Where the `vDSO` clock mode can be one of the:
|
||||
|
||||
```C
|
||||
@ -190,7 +190,7 @@ for the `x86` architectures. Where the `vDSO` clock mode can be one of the:
|
||||
#define VCLOCK_PVCLOCK 3
|
||||
```
|
||||
|
||||
The last three fields are `wd_list`, `cs_last` and the `wd_last` depends on the `CONFIG_CLOCKSOURCE_WATCHDOG` kernel configuration option. First of all let's try to understand what is it `watchdog`. In a simple words, watchdog is a timer that is used for detection of the computer malfunctions and recovering from it. All of these three fields contain watchdog related data that is used by the `clocksource` framework. If we will grep the Linux kernel source code, we will see that only [arch/x86/KConfig](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/arch/x86/Kconfig#L54) kernel configuration file contains the `CONFIG_CLOCKSOURCE_WATCHDOG` kernel configuration option. So, why do `x86` and `x86_64` need in [watchdog](https://en.wikipedia.org/wiki/Watchdog_timer)? You already may know that all `x86` processors has special 64-bit register - [time stamp counter](https://en.wikipedia.org/wiki/Time_Stamp_Counter). This register contains number of [cycles](https://en.wikipedia.org/wiki/Clock_rate) since the reset. Sometimes the time stamp counter needs to be verified against another clock source. We will not see initialization of the `watchdog` timer in this part, before this we must learn more about timers.
|
||||
The last three fields are `wd_list`, `cs_last` and the `wd_last` depends on the `CONFIG_CLOCKSOURCE_WATCHDOG` kernel configuration option. First of all let's try to understand what is `watchdog`. In a simple words, watchdog is a timer that is used for detection of the computer malfunctions and recovering from it. All of these three fields contain watchdog related data that is used by the `clocksource` framework. If we will grep the Linux kernel source code, we will see that only [arch/x86/KConfig](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/arch/x86/Kconfig#L54) kernel configuration file contains the `CONFIG_CLOCKSOURCE_WATCHDOG` kernel configuration option. So, why do `x86` and `x86_64` need in [watchdog](https://en.wikipedia.org/wiki/Watchdog_timer)? You already may know that all `x86` processors has special 64-bit register - [time stamp counter](https://en.wikipedia.org/wiki/Time_Stamp_Counter). This register contains number of [cycles](https://en.wikipedia.org/wiki/Clock_rate) since the reset. Sometimes the time stamp counter needs to be verified against another clock source. We will not see initialization of the `watchdog` timer in this part, before this we must learn more about timers.
|
||||
|
||||
That's all. From this moment we know all fields of the `clocksource` structure. This knowledge will help us to learn insides of the `clocksource` framework.
|
||||
|
||||
@ -241,9 +241,9 @@ int __clocksource_register_scale(struct clocksource *cs, u32 scale, u32 freq)
|
||||
}
|
||||
```
|
||||
|
||||
First of all we can see that the `__clocksource_register_scale` function starts from the call of the `__clocksource_update_freq_scale` function that defined in the same source code file and updates given clock source with the new frequency. Let's look on the implementation of this function. In the first step we need to check given frequency and if it was not passed as `zero`, we need to calculate `mult` and `shift` parameters for the given clock source. Why do we need to check value of the `frequency`? Actually it can be zero. if you attentively looked on the implementation of the `__clocksource_register` function, you may have noticed that we passed `frequency` as `0`. We will do it only for some clock sources that have self defined `mult` and `shift` parameters. Look in the previous [part](https://0xax.gitbook.io/linux-insides/summary/timers/linux-timers-1) and you will see that we saw calculation of the `mult` and `shift` for `jiffies`. The `__clocksource_update_freq_scale` function will do it for us for other clock sources.
|
||||
First of all we can see that the `__clocksource_register_scale` function starts from the call of the `__clocksource_update_freq_scale` function that defined in the same source code file and updates given clock source with the new frequency. Let's look on the implementation of this function. In the first step we need to check given frequency and if it was not passed as `zero`, we need to calculate `mult` and `shift` parameters for the given clock source. Why do we need to check value of the `frequency`? Actually it can be zero. If you attentively looked on the implementation of the `__clocksource_register` function, you may have noticed that we passed `frequency` as `0`. We will do it only for some clock sources that have self defined `mult` and `shift` parameters. Look in the previous [part](https://0xax.gitbook.io/linux-insides/summary/timers/linux-timers-1) and you will see that we saw calculation of the `mult` and `shift` for `jiffies`. The `__clocksource_update_freq_scale` function will do it for us for other clock sources.
|
||||
|
||||
So in the start of the `__clocksource_update_freq_scale` function we check the value of the `frequency` parameter and if is not zero we need to calculate `mult` and `shift` for the given clock source. Let's look on the `mult` and `shift` calculation:
|
||||
So in the start of the `__clocksource_update_freq_scale` function we check the value of the `frequency` parameter and if it is not zero we need to calculate `mult` and `shift` for the given clock source. Let's look on the `mult` and `shift` calculation:
|
||||
|
||||
```C
|
||||
void __clocksource_update_freq_scale(struct clocksource *cs, u32 scale, u32 freq)
|
||||
@ -259,7 +259,7 @@ void __clocksource_update_freq_scale(struct clocksource *cs, u32 scale, u32 freq
|
||||
sec = 1;
|
||||
else if (sec > 600 && cs->mask > UINT_MAX)
|
||||
sec = 600;
|
||||
|
||||
|
||||
clocks_calc_mult_shift(&cs->mult, &cs->shift, freq,
|
||||
NSEC_PER_SEC / scale, sec * scale);
|
||||
}
|
||||
@ -406,21 +406,21 @@ and creation of three files:
|
||||
|
||||
These files will provide information about current clock source in the system, available clock sources in the system and interface which allows to unbind the clock source.
|
||||
|
||||
After the `init_clocksource_sysfs` function will be executed, we will be able find some information about available clock sources in the:
|
||||
After the `init_clocksource_sysfs` function is executed, we will be able to find some information about available clock sources in the:
|
||||
|
||||
```
|
||||
$ cat /sys/devices/system/clocksource/clocksource0/available_clocksource
|
||||
tsc hpet acpi_pm
|
||||
$ cat /sys/devices/system/clocksource/clocksource0/available_clocksource
|
||||
tsc hpet acpi_pm
|
||||
```
|
||||
|
||||
Or for example information about current clock source in the system:
|
||||
|
||||
```
|
||||
$ cat /sys/devices/system/clocksource/clocksource0/current_clocksource
|
||||
$ cat /sys/devices/system/clocksource/clocksource0/current_clocksource
|
||||
tsc
|
||||
```
|
||||
|
||||
In the previous part, we saw API for the registration of the `jiffies` clock source, but didn't dive into details about the `clocksource` framework. In this part we did it and saw implementation of the new clock source registration and selection of a clock source with the best rating value in the system. Of course, this is not all API that `clocksource` framework provides. There a couple additional functions like `clocksource_unregister` for removing given clock source from the `clocksource_list` and etc. But I will not describe this functions in this part, because they are not important for us right now. Anyway if you are interesting in it, you can find it in the [kernel/time/clocksource.c](https://github.com/torvalds/linux/tree/master/kernel/time/clocksource.c).
|
||||
In the previous part, we saw API for the registration of the `jiffies` clock source, but didn't dive into details about the `clocksource` framework. In this part we did it and saw implementation of the new clock source registration and selection of a clock source with the best rating value in the system. Of course, this is not all API that `clocksource` framework provides. There a couple additional functions like `clocksource_unregister` for removing given clock source from the `clocksource_list` and etc. But I will not describe this functions in this part, because they are not important for us right now. Anyway if you are interested in it, you can find it in the [kernel/time/clocksource.c](https://github.com/torvalds/linux/tree/master/kernel/time/clocksource.c).
|
||||
|
||||
That's all.
|
||||
|
||||
|
||||
@ -4,17 +4,17 @@ Timers and time management in the Linux kernel. Part 3.
|
||||
The tick broadcast framework and dyntick
|
||||
--------------------------------------------------------------------------------
|
||||
|
||||
This is third part of the [chapter](https://0xax.gitbook.io/linux-insides/summary/timers/) which describes timers and time management related stuff in the Linux kernel and we stopped on the `clocksource` framework in the previous [part](https://0xax.gitbook.io/linux-insides/summary/timers/linux-timers-2). We have started to consider this framework because it is closely related to the special counters which are provided by the Linux kernel. One of these counters which we already saw in the first [part](https://0xax.gitbook.io/linux-insides/summary/timers/linux-timers-1) of this chapter is - `jiffies`. As I already wrote in the first part of this chapter, we will consider time management related stuff step by step during the Linux kernel initialization. Previous step was call of the:
|
||||
This is third part of the [chapter](https://0xax.gitbook.io/linux-insides/summary/timers/) which describes timers and time management related stuff in the Linux kernel and we stopped on the `clocksource` framework in the previous [part](https://0xax.gitbook.io/linux-insides/summary/timers/linux-timers-2). We have started to consider this framework because it is closely related to the special counters which are provided by the Linux kernel. One of these counters which we already saw in the first [part](https://0xax.gitbook.io/linux-insides/summary/timers/linux-timers-1.md) of this chapter is - `jiffies`. As I already wrote in the first part of this chapter, we will consider time management related stuff step by step during the Linux kernel initialization. Previous step was call of the:
|
||||
|
||||
```C
|
||||
register_refined_jiffies(CLOCK_TICK_RATE);
|
||||
```
|
||||
|
||||
function which defined in the [kernel/time/jiffies.c](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/kernel/time/jiffies.c) source code file and executes initialization of the `refined_jiffies` clock source for us. Recall that this function is called from the `setup_arch` function that defined in the [arch/x86/kernel/setup.c](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/arch/x86/kernel/setup.c) source code and executes architecture-specific ([x86_64](https://en.wikipedia.org/wiki/X86-64) in our case) initialization. Look on the implementation of the `setup_arch` and you will note that the call of the `register_refined_jiffies` is the last step before the `setup_arch` function will finish its work.
|
||||
function which is defined in the [kernel/time/jiffies.c](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/kernel/time/jiffies.c) source code file and executes initialization of the `refined_jiffies` clock source for us. Recall that this function is called from the `setup_arch` function that is defined in the [arch/x86/kernel/setup.c](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/arch/x86/kernel/setup.c) source code and executes architecture-specific ([x86_64](https://en.wikipedia.org/wiki/X86-64) in our case) initialization. Look on the implementation of the `setup_arch` and you will note that the call of the `register_refined_jiffies` is the last step before the `setup_arch` function finishes its work.
|
||||
|
||||
There are many different `x86_64` specific things already configured after the end of the `setup_arch` execution. For example some early [interrupt](https://en.wikipedia.org/wiki/Interrupt) handlers already able to handle interrupts, memory space reserved for the [initrd](https://en.wikipedia.org/wiki/Initrd), [DMI](https://en.wikipedia.org/wiki/Desktop_Management_Interface) scanned, the Linux kernel log buffer is already set and this means that the [printk](https://en.wikipedia.org/wiki/Printk) function is able to work, [e820](https://en.wikipedia.org/wiki/E820) parsed and the Linux kernel already knows about available memory and and many many other architecture specific things (if you are interesting, you can read more about the `setup_arch` function and Linux kernel initialization process in the second [chapter](https://0xax.gitbook.io/linux-insides/summary/initialization) of this book).
|
||||
There are many different `x86_64` specific things already configured after the end of the `setup_arch` execution. For example some early [interrupt](https://en.wikipedia.org/wiki/Interrupt) handlers already able to handle interrupts, memory space reserved for the [initrd](https://en.wikipedia.org/wiki/Initrd), [DMI](https://en.wikipedia.org/wiki/Desktop_Management_Interface) scanned, the Linux kernel log buffer is already set and this means that the [printk](https://en.wikipedia.org/wiki/Printk) function is able to work, [e820](https://en.wikipedia.org/wiki/E820) parsed and the Linux kernel already knows about available memory and and many many other architecture specific things (if you are interested, you can read more about the `setup_arch` function and Linux kernel initialization process in the second [chapter](https://0xax.gitbook.io/linux-insides/summary/initialization) of this book).
|
||||
|
||||
Now, the `setup_arch` finished its work and we can back to the generic Linux kernel code. Recall that the `setup_arch` function was called from the `start_kernel` function which is defined in the [init/main.c](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/init/main.c) source code file. So, we shall return to this function. You can see that there are many different function are called right after `setup_arch` function inside of the `start_kernel` function, but since our chapter is devoted to timers and time management related stuff, we will skip all code which is not related to this topic. The first function which is related to the time management in the Linux kernel is:
|
||||
Now, the `setup_arch` finished its work and we can go back to the generic Linux kernel code. Recall that the `setup_arch` function was called from the `start_kernel` function which is defined in the [init/main.c](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/init/main.c) source code file. So, we shall return to this function. You can see that there are many different functions that are called right after `setup_arch` function inside of the `start_kernel` function, but since our chapter is devoted to timers and time management related stuff, we will skip all code which is not related to this topic. The first function which is related to the time management in the Linux kernel is:
|
||||
|
||||
```C
|
||||
tick_init();
|
||||
@ -25,12 +25,12 @@ in the `start_kernel`. The `tick_init` function defined in the [kernel/time/tick
|
||||
* Initialization of `tick broadcast` framework related data structures;
|
||||
* Initialization of `full` tickless mode related data structures.
|
||||
|
||||
We didn't see anything related to the `tick broadcast` framework in this book and didn't know anything about tickless mode in the Linux kernel. So, the main point of this part is to look on these concepts and to know what are they.
|
||||
We didn't see anything related to the `tick broadcast` framework in this book and didn't know anything about tickless mode in the Linux kernel. So, the main point of this part is to look on these concepts and to know what they are.
|
||||
|
||||
The idle process
|
||||
--------------------------------------------------------------------------------
|
||||
|
||||
First of all, let's look on the implementation of the `tick_init` function. As I already wrote, this function defined in the [kernel/time/tick-common.c](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/kernel/time/tick-common.c) source code file and consists from the two calls of following functions:
|
||||
First of all, let's look on the implementation of the `tick_init` function. As I already wrote, this function is defined in the [kernel/time/tick-common.c](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/kernel/time/tick-common.c) source code file and consists from the two calls of following functions:
|
||||
|
||||
```C
|
||||
void __init tick_init(void)
|
||||
@ -51,16 +51,17 @@ static void cpu_idle_loop(void)
|
||||
{
|
||||
while (1) {
|
||||
while (!need_resched()) {
|
||||
...
|
||||
...
|
||||
...
|
||||
/* the main idle function */
|
||||
cpuidle_idle_call();
|
||||
}
|
||||
...
|
||||
...
|
||||
...
|
||||
/* the main idle function */
|
||||
cpuidle_idle_call();
|
||||
schedule_preempt_disabled();
|
||||
}
|
||||
...
|
||||
...
|
||||
...
|
||||
schedule_preempt_disabled();
|
||||
}
|
||||
```
|
||||
|
||||
@ -74,7 +75,7 @@ Whenever the idle task is selected to run, the periodic tick is disabled with th
|
||||
|
||||
The second way is to omit scheduling-clock ticks on processors that are either in `idle` state or that have only one runnable task or in other words busy processor. We can enable this feature with the `CONFIG_NO_HZ_FULL` kernel configuration option and it allows to reduce the number of timer interrupts significantly.
|
||||
|
||||
Besides the `cpu_idle_loop`, idle processor can be in a sleeping state. The Linux kernel provides special `cpuidle` framework. Main point of this framework is to put an idle processor to sleeping states. The name of the set of these states is - `C-states`. But how does a processor will be woken if local timer is disabled? The Linux kernel provides `tick broadcast` framework for this. The main point of this framework is assign a timer which is not affected by the `C-states`. This timer will wake a sleeping processor.
|
||||
Besides the `cpu_idle_loop`, idle processor can be in a sleeping state. The Linux kernel provides special `cpuidle` framework. Main point of this framework is to put an idle processor to sleeping states. The name of the set of these states is - `C-states`. But how will a processor will be woken if local timer is disabled? The Linux kernel provides `tick broadcast` framework for this. The main point of this framework is assign a timer which is not affected by the `C-states`. This timer will wake a sleeping processor.
|
||||
|
||||
Now, after some theory we can return to the implementation of our function. Let's recall that the `tick_init` function just calls two following functions:
|
||||
|
||||
@ -117,7 +118,7 @@ Ultimately, the memory space will be allocated for the given `cpumask` with the
|
||||
*mask = kmalloc_node(cpumask_size(), flags, node);
|
||||
```
|
||||
|
||||
Now let's look on the `cpumasks` that will be initialized in the `tick_broadcast_init` function. As we can see, the `tick_broadcast_init` function will initialize six `cpumasks`, and moreover, initialization of the last three `cpumasks` will be depended on the `CONFIG_TICK_ONESHOT` kernel configuration option.
|
||||
Now let's look on the `cpumasks` that will be initialized in the `tick_broadcast_init` function. As we can see, the `tick_broadcast_init` function will initialize six `cpumasks`, and moreover, initialization of the last three `cpumasks` will depend on the `CONFIG_TICK_ONESHOT` kernel configuration option.
|
||||
|
||||
The first three `cpumasks` are:
|
||||
|
||||
@ -157,7 +158,7 @@ struct tick_device {
|
||||
};
|
||||
```
|
||||
|
||||
Note, that the `tick_device` structure contains two fields. The first field - `evtdev` represents pointer to the `clock_event_device` structure that defined in the [include/linux/clockchips.h](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/include/linux/clockchips.h) header file and represents descriptor of a clock event device. A `clock event` device allows to register an event that will happen in the future. As I already wrote, we will not consider `clock_event_device` structure and related API in this part, but will see it in the next part.
|
||||
Note, that the `tick_device` structure contains two fields. The first field - `evtdev` represents pointer to the `clock_event_device` structure that is defined in the [include/linux/clockchips.h](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/include/linux/clockchips.h) header file and represents descriptor of a clock event device. A `clock event` device allows to register an event that will happen in the future. As I already wrote, we will not consider `clock_event_device` structure and related API in this part, but will see it in the next part.
|
||||
|
||||
The second field of the `tick_device` structure represents mode of the `tick_device`. As we already know, the mode can be one of the:
|
||||
|
||||
@ -208,7 +209,7 @@ First of all we get the current `clock event` device from the `tick_broadcast_de
|
||||
static struct tick_device tick_broadcast_device;
|
||||
```
|
||||
|
||||
and represents external clock device that keeps track of events for a processor. The first step after we got the current clock device is the call of the `tick_check_broadcast_device` function which checks that a given clock events device can be utilized as broadcast device. The main point of the `tick_check_broadcast_device` function is to check value of the `features` field of the given `clock events` device. As we can understand from the name of this field, the `features` field contains a clock event device features. Available values defined in the [include/linux/clockchips.h](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/include/linux/clockchips.h) header file and can be one of the `CLOCK_EVT_FEAT_PERIODIC` - which represents a clock events device which supports periodic events and etc. So, the `tick_check_broadcast_device` function check `features` flags for `CLOCK_EVT_FEAT_ONESHOT`, `CLOCK_EVT_FEAT_DUMMY` and other flags and returns `false` if the given clock events device has one of these features. In other way the `tick_check_broadcast_device` function compares `ratings` of the given clock event device and current clock event device and returns the best.
|
||||
and represents external clock device that keeps track of events for a processor. The first step after we get the current clock device is the call of the `tick_check_broadcast_device` function which checks that a given clock events device can be utilized as broadcast device. The main point of the `tick_check_broadcast_device` function is to check value of the `features` field of the given `clock events` device. As we can understand from the name of this field, the `features` field contains a clock event device features. Available values defined in the [include/linux/clockchips.h](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/include/linux/clockchips.h) header file and can be one of the `CLOCK_EVT_FEAT_PERIODIC` - which represents a clock events device which supports periodic events and etc. So, the `tick_check_broadcast_device` function check `features` flags for `CLOCK_EVT_FEAT_ONESHOT`, `CLOCK_EVT_FEAT_DUMMY` and other flags and returns `false` if the given clock events device has one of these features. In other way the `tick_check_broadcast_device` function compares `ratings` of the given clock event device and current clock event device and returns the best.
|
||||
|
||||
After the `tick_check_broadcast_device` function, we can see the call of the `try_module_get` function that checks module owner of the clock events. We need to do it to be sure that the given `clock events` device was correctly initialized. The next step is the call of the `clockevents_exchange_device` function that defined in the [kernel/time/clockevents.c](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/kernel/time/clockevents.c) source code file and will release old clock events device and replace the previous functional handler with a dummy handler.
|
||||
|
||||
@ -292,7 +293,7 @@ static irqreturn_t hpet_interrupt_handler(int irq, void *data)
|
||||
}
|
||||
```
|
||||
|
||||
The `hpet_interrupt_handler` gets the [irq](https://en.wikipedia.org/wiki/Interrupt_request_%28PC_architecture%29) specific data and check the event handler of the `clock event` device. Recall that we just set in the `tick_set_periodic_handler` function. So the `tick_handler_periodic_broadcast` function will be called in the end of the high precision event timer interrupt handler.
|
||||
The `hpet_interrupt_handler` gets the [IRQ](https://en.wikipedia.org/wiki/Interrupt_request_%28PC_architecture%29) specific data and check the event handler of the `clock event` device. Recall that we just set in the `tick_set_periodic_handler` function. So the `tick_handler_periodic_broadcast` function will be called in the end of the high precision event timer interrupt handler.
|
||||
|
||||
The `tick_handler_periodic_broadcast` function calls the
|
||||
|
||||
@ -314,7 +315,7 @@ if (bc_local)
|
||||
td->evtdev->event_handler(td->evtdev);
|
||||
```
|
||||
|
||||
which actually represents interrupt handler of the local timer of a processor. After this a processor will wake up. That is all about `tick broadcast` framework in the Linux kernel. We have missed some aspects of this framework, for example reprogramming of a `clock event` device and broadcast with the oneshot timer and etc. But the Linux kernel is very big, it is not real to cover all aspects of it. I think it will be interesting to dive into with yourself.
|
||||
which actually represents interrupt handler of the local timer of a processor. After this a processor will wake up. That is all about `tick broadcast` framework in the Linux kernel. We have missed some aspects of this framework, for example reprogramming of a `clock event` device and broadcast with the oneshot timer and etc. But the Linux kernel is very big, it is not realistic to cover all aspects of it. I think it will be interesting to dive into it yourself.
|
||||
|
||||
If you remember, we have started this part with the call of the `tick_init` function. We just consider the `tick_broadcast_init` function and related theory, but the `tick_init` function contains another call of a function and this function is - `tick_nohz_init`. Let's look on the implementation of this function.
|
||||
|
||||
@ -435,7 +436,7 @@ Links
|
||||
* [NO_HZ documentation](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/Documentation/timers/NO_HZ.txt)
|
||||
* [cpumasks](https://0xax.gitbook.io/linux-insides/summary/concepts/linux-cpu-2)
|
||||
* [high precision event timer](https://en.wikipedia.org/wiki/High_Precision_Event_Timer)
|
||||
* [irq](https://en.wikipedia.org/wiki/Interrupt_request_%28PC_architecture%29)
|
||||
* [IRQ](https://en.wikipedia.org/wiki/Interrupt_request_%28PC_architecture%29)
|
||||
* [IPI](https://en.wikipedia.org/wiki/Inter-processor_interrupt)
|
||||
* [CPUID](https://en.wikipedia.org/wiki/CPUID)
|
||||
* [APIC](https://en.wikipedia.org/wiki/Advanced_Programmable_Interrupt_Controller)
|
||||
|
||||
@ -134,4 +134,6 @@ Thank you to all contributors:
|
||||
* [Yuxin Wu](https://github.com/chaffz)
|
||||
* [Biao Ding](https://github.com/SmallPond)
|
||||
* [Arfy slowy](https://github.com/slowy07)
|
||||
* [Junbo Jiang](https://github.com/junbo42)
|
||||
* [Dexter Plameras](https://github.com/dexterp)
|
||||
* [Jun Duan](https://github.com/waltforme)
|
||||
|
||||
Loading…
Reference in New Issue
Block a user