As you may understand from the title, this part will cover interesting and important concept in the Linux kernel which is called - `initcall`. We already saw definitions like these:
in some parts of the Linux kernel. Before we see how this mechanism is implemented in the Linux kernel, we must know actually what is it and how the Linux kernel uses it. Definitions like these represent a [callback](https://en.wikipedia.org/wiki/Callback_%28computer_programming%29) function which will be called during initialization of the Linux kernel or right after it. Actually the main point of the `initcall` mechanism is to determine correct order of the built-in modules and subsystems initialization. For example let's look at the following function:
from the [arch/x86/kernel/nmi.c](https://github.com/torvalds/linux/blob/master/arch/x86/kernel/nmi.c) source code file. As we may see it just creates the `nmi_longest_ns` [debugfs](https://en.wikipedia.org/wiki/Debugfs) file in the `arch_debugfs_dir` directory. Actually, this `debugfs` file may be created only after the `arch_debugfs_dir` will be created. Creation of this directory occurs during the architecture-specific initalization of the Linux kernel. Actually this directory will be created in the `arch_kdebugfs_init` function from the [arch/x86/kernel/kdebugfs.c](https://github.com/torvalds/linux/blob/master/arch/x86/kernel/kdebugfs.c) source code file. Note that the `arch_kdebugfs_init` function is marked as `initcall` too:
```C
arch_initcall(arch_kdebugfs_init);
```
The Linux kernel calls all architecture-specific `initcalls` before the `fs` related `initicalls`. So, our `nmi_longest_ns` file will be created only after the `arch_kdebugfs_dir` directory will be created. Actually, the Linux kernel provides eight levels of main `initcalls`:
All of their names are represented by the `initcall_level_names` array which is defined in the [init/main.c](https://github.com/torvalds/linux/blob/master/init/main.c) source code file:
All functions which are marked as `initcall` by these identificators, will be called in the same order or at first `early initcalls` will be called, at second `core initcalls` and etc. From this moment we know a little about `initcall` mechanism, so we can start to dive into the source code of the Linux kernel to see how this mechanism is implemented.
The Linux kernel provides a set of macros from the [include/linux/init.h](https://github.com/torvalds/linux/blob/master/include/linux/init.h) header file to mark a given function as `initicall`. All of these macros are pretty simple:
and as we may see these macros just expands to the call of the `__define_initcall` macro from the same header file. As we may see, the `__define_inticall` macro takes two arguments:
To understand the `__define_initcall` macro, first of all let's look at the `initcall_t` type. This type is defined in the same [header]() file and represents pointer to a function which returns pointer to [integer](https://en.wikipedia.org/wiki/Integer) which will be result of the `initcall`:
Now let's return to the `_-define_initicall` macro. The [##](https://gcc.gnu.org/onlinedocs/cpp/Concatenation.html) provides ability to concatenate two symbols. In our case, the first line of the `__define_initcall` macro produces definition of the given function which is located in the `.initcall id .init` [ELF section](http://www.skyfree.org/linux/references/ELF_Format.pdf) and makred with the following [gcc](https://en.wikipedia.org/wiki/GNU_Compiler_Collection) attributes: `__initicall_function_name_id` and `__used`. If we will look in the [include/asm-generic/vmlinux.lds.h](https://github.com/torvalds/linux/blob/master/include/asm-generic/vmlinux.lds.h) header file which represents data for the kernel [linker](https://en.wikipedia.org/wiki/Linker_%28computing%29) script, we will see that all of `initcalls` sections will be placed in the `.data` section:
The seconds attribute - `__used` is defined in the [include/linux/compiler-gcc.h](https://github.com/torvalds/linux/blob/master/include/linux/compiler-gcc.h) header file and just expands to the definition of the following `gcc` attribute:
```C
#define __used __attribute__((__used__))
```
which prevents `variable defined but not used` warning. The last line of the `__define_initcall` macro is:
depends on the `CONFIG_LTO` kernel configuration option and just provides stub for the compiler [Link time optimization](https://gcc.gnu.org/wiki/LinkTimeOptimization):
to prevent problem when there is no reference to a variable in a module it will be moved to the end of the program. That's all about the `__define_initcall` macro. So, all of the `*_initcall` macros will be expanded during compilation of the Linux kernel, and all `initcalls` will be placed in their sections and all of them will be available from the `.data` section and the Linux kernel will know where to find a certain `initcall` to call it during initialization process.
As `initcalls` can be called by the Linux kernel, let's look how the Linux kernel does this. This process starts in the `do_basic_setup` function from the [init/main.c](https://github.com/torvalds/linux/blob/master/init/main.c) source code file:
```C
static void __init do_basic_setup(void)
{
...
...
...
do_initcalls();
...
...
...
}
```
which is called during the initialization of the Linux kernel, right after main steps of initialization like memory manager related initializations, `CPU` subsystem and other already finished. The `do_initcalls` function just goes through the array of `initcall` levels and call the `do_initcall_level` function for each level:
```C
static void __init do_initcalls(void)
{
int level;
for (level = 0; level <ARRAY_SIZE(initcall_levels)-1;level++)
do_initcall_level(level);
}
```
The `initcall_levels` array is defined in the same source code [file](https://github.com/torvalds/linux/blob/master/init/main.c) and contains pointers to the sections which were defined in the `__define_initcall` macro:
If you are interested, you can find these sections in the `arch/x86/kernel/vmlinux.lds` linker script which is generated after the Linux kernel compilation:
If this is not familar for you, you can know more about [linkers](https://en.wikipedia.org/wiki/Linker_%28computing%29) in the special [part](https://0xax.gitbooks.io/linux-insides/content/Misc/linkers.html) of this book.
As we just saw, the `do_initcall_level` function takes one parameter - level of `initcall` and does two following thigs: First of all this function parses the `initcall_command_line` which is copy of usual kernel [command line](https://www.kernel.org/doc/Documentation/kernel-parameters.txt) which may contain parameters for modules with the `parse_args` function from the [kernel/params.c](https://github.com/torvalds/linux/blob/master/kernel/params.c) source code file and call the `do_on_initcall` function for each level:
```C
for (fn = initcall_levels[level]; fn <initcall_levels[level+1];fn++)
do_one_initcall(*fn);
```
The `do_on_initcall` does all main job for us. As we may see, this function takes one paraemter which represent `initcall` callback function and does the call of the given callback:
```C
int __init_or_module do_one_initcall(initcall_t fn)
Let's try to understand what does the `do_on_initcall` function does. First of all we increase [preemption](https://en.wikipedia.org/wiki/Preemption_%28computing%29) counter to check it later to be sure that it is not imbalanced. After this step we can see the call of the `initcall_backlist` function which
The blacklisted `initcalls` stored in the `blacklisted_initcalls` list and this list is filled during early Linux kernel initialization from the Linux kernel command line.
After the blakclisted `initcalls` will be handled, the next part of code does directly the call of the `initcall`:
```C
if (initcall_debug)
ret = do_one_initcall_debug(fn);
else
ret = fn();
```
Depends on the valule of the `initcall_debug` variable, the `do_one_initcall_debug` function will call `initcall` or this function will do it directly via `fn()`. The `initcall_debug` variable is defined in the [same](https://github.com/torvalds/linux/blob/master/init/main.c) source code file:
and provides ability to print some information to the kernel [log buffer](https://en.wikipedia.org/wiki/Dmesg). The value of the variable can be set from the kernel commands via the `initcall_debug` parameter. As we can read from the [documentation](https://www.kernel.org/doc/Documentation/kernel-parameters.txt) of the Linux kernel command line:
initcall_debug [KNL] Trace initcalls as they are executed. Useful
for working out where the kernel is dying during
startup.
```
And that's true. If we will look at the implementation of the `do_one_initcall_debug` function, we will see that it does the same as the `do_one_initcall` function or i.e. the `do_one_initcall_debug` function calls the given `initcall` and prints some information (like the [pid](https://en.wikipedia.org/wiki/Process_identifier) of the currently running task, duration of execution of the `initcall` and etc.) related to the execution of the given `initcall`:
```C
static int __init_or_module do_one_initcall_debug(initcall_t fn)
As an `initcall` was called by the one of the ` do_one_initcall` or `do_one_initcall_debug` functions, we may see two checks in the end of the `do_one_initcall` function. The first one checks the amount of possible `__preempt_count_add` and `__preempt_count_sub` calls inside of the executed initcall, and if this value is not equal to the previous value of the preemptible counter, we add the `preemption imbalance` string to the message buffer and set correct value of the preemptible counter:
Later this error string will be printed. The last check the state of local [IRQs](https://en.wikipedia.org/wiki/Interrupt_request_%28PC_architecture%29) and if they are disabled, we add the `disabled interrupts` strings to the our message buffer and enable `IRQs` for the current processor to prevent the state when `IRQs` were disabled by an `initcall` and didn't enabled again:
That's all. In this way the Linux kernel does initialization of many subsystems in a correct order. From now we know what is it `initcall` mechanism in the Linux kernel. We saw main general part of the `initcall` mechanism in this part. But we avoided some important concepts. Let's make a short look at these concepts.
First of all, we have missed one level of `initcalls`, this is `rootfs initcalls`. You can find definition of the `rootfs_initcall` in the [include/linux/init.h](https://github.com/torvalds/linux/blob/master/include/linux/init.h) header file together with all similar macros which we saw in this part:
As we may understand from the macro's name, its main purpose is to store callbacks which are related to the [rootfs](https://en.wikipedia.org/wiki/Initramfs). Besides this goal, it may be useful to initialize other stuffs after initialization related to filesystems level, but only before devices related stuff are not initialized. For example, the decompression of the [initramfs](https://en.wikipedia.org/wiki/Initramfs) which occurred in the `populate_rootfs` function from the [init/initramfs.c](https://github.com/torvalds/linux/blob/master/init/initramfs.c) source code file:
Besides the `rootfs_initcall` level, there are additional `console_initcall`, `security_initcall` and other secondary `initcall` levels. The last thing that we have missed is the set of the `*_initcall_sync` levels. Almost each `*_initcall` macro that we have seen in this part, has macro companion with the `_sync` prefix:
In this part we saw the important mechanism of the Linux kernel which allows to call a function which depends on the current state of the Linux kernel during its initialization.
If you have questions or suggestions, feel free to ping me in twitter [0xAX](https://twitter.com/0xAX), drop me [email](anotherworldofworld@gmail.com) or just create [issue](https://github.com/0xAX/linux-insides/issues/new).
**Please note that English is not my first language and I am really sorry for any inconvenience. If you found any mistakes please send me PR to [linux-insides](https://github.com/0xAX/linux-insides).**.
