@ -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.
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).
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:
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).
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`.
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:
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 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:
```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)) {
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:
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 wakeup related process:
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 `
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:
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:
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`:
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, 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.
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).
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:
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.
Renaud Germain
2 years ago