@ -135,7 +135,7 @@ Let's look at the operations that can performed on a spinlock:
* `spin_is_locked` - returns the state of the given `spinlock`;
* and etc.
and the implementation of the `spin_lock_init` macro (from include/linux/spinlock.h](https://github.com/torvalds/linux/master/include/linux/spinlock.h)):
and the implementation of the `spin_lock_init` macro (from [include/linux/spinlock.h](https://github.com/torvalds/linux/master/include/linux/spinlock.h)):
The idea is simple, but the implementation of the `queued spinlocks` is must complex than this pseudocode. As I already wrote above, the `queued spinlock` mechanism is planned to be replacement for `ticket spinlocks` in the Linux kernel. But as you may remember, the usual `spinlock` fit into `32-bit` [word](https://en.wikipedia.org/wiki/Word_%28computer_architecture%29). But the `MCS` based lock does not fit to this size. As you may know `spinlock_t` type is [widely](http://lxr.free-electrons.com/ident?i=spinlock_t) used in the Linux kernel. In this case would have to rewrite a significant part of the Linux kernel, but this is unacceptable. Beside this, some kernel structures which contains a spinlock for protection can't grow. But anyway, implementation of the `queued spinlocks` in the Linux kernel based on this concept with some modifications which allows to fit it into `32` bits.
The idea is simple, but the implementation of the `queued spinlocks` is more complex than this pseudocode. As I already wrote above, the `queued spinlock` mechanism is planned to be replacement for `ticket spinlocks` in the Linux kernel. But as you may remember, the usual `spinlock` fit into `32-bit` [word](https://en.wikipedia.org/wiki/Word_%28computer_architecture%29). But the `MCS` based lock does not fit to this size. As you may know `spinlock_t` type is [widely](http://lxr.free-electrons.com/ident?i=spinlock_t) used in the Linux kernel. In this case would have to rewrite a significant part of the Linux kernel, but this is unacceptable. Beside this, some kernel structures which contains a spinlock for protection can't grow. But anyway, implementation of the `queued spinlocks` in the Linux kernel based on this concept with some modifications which allows to fit it into `32` bits.
That's all about theory of the `queued spinlocks`, now let's consider how this mechanism is implemented in the Linux kernel. Implementation of the `queued spinlocks` looks more complex and tangled than implementation of `ticket spinlocks`, but the study with attention will lead to success.
Let's consider implementation of this function. It looks pretty easy and actually it does almost the same. Thus function executes initialization of the given `semaphore` with the `__SEMAPHORE_INITIALIZER` macro which we just saw. As I already wrote in the previous parts of this [chapter](https://0xax.gitbooks.io/linux-insides/content/SyncPrim/index.html), we will skip the stuff which is related to the [lock validator](https://www.kernel.org/doc/Documentation/locking/lockdep-design.txt) of the Linux kernel.
Let's consider implementation of this function. It looks pretty easy and actually it does almost the same. This function executes initialization of the given `semaphore` with the `__SEMAPHORE_INITIALIZER` macro which we just saw. As I already wrote in the previous parts of this [chapter](https://0xax.gitbooks.io/linux-insides/content/SyncPrim/index.html), we will skip the stuff which is related to the [lock validator](https://www.kernel.org/doc/Documentation/locking/lockdep-design.txt) of the Linux kernel.
So, from now we are able to initialize a `semaphore` let's look at how to lock and unlock. The Linux kernel provides following [API](https://en.wikipedia.org/wiki/Application_programming_interface) to manipulate `semaphores`:
@ -102,7 +102,7 @@ int down_trylock(struct semaphore *sem);
int down_timeout(struct semaphore *sem, long jiffies);
```
The first two functions: `down` and `up` are for acquiring and releasing of the given `semaphore`. The `down_interruptible` function tries to acquire a `semaphore`. If this try was successful, the `count` field of the given `semaphore` will be decremented and lock will be acquired, in other way the task will be switched to the blocked state or in other words the `TASK_INTERRUPTIBLE` flag will be set. This `TASK_INTERRUPTIBLE` flag means that the process may returned to ruined state by [signal](https://en.wikipedia.org/wiki/Unix_signal).
The first two functions: `down` and `up` are for acquiring and releasing of the given `semaphore`. The `down_interruptible` function tries to acquire a `semaphore`. If this try was successful, the `count` field of the given `semaphore` will be decremented and lock will be acquired, in other way the task will be switched to the blocked state or in other words the `TASK_INTERRUPTIBLE` flag will be set. This `TASK_INTERRUPTIBLE` flag means that the process may return to running state by a [signal](https://en.wikipedia.org/wiki/Unix_signal).
The `down_killable` function does the same as the `down_interruptible` function, but set the `TASK_KILLABLE` flag for the current process. This means that the waiting process may be interrupted by the kill signal.
@ -53,7 +53,7 @@ The next two fields of the `mutex` structure - `wait_lock` and `wait_list` are [
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 which wants to acquire a lock, must decrease value of the `mutex->count` if possible. And if a process wants to release a lock, it must 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 will not 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/master/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.gitbooks.io/linux-insides/content/SyncPrim/sync-3.html) 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/master/kernel/locking/semaphore.c) source code file:
structure from the [include/linux/mutex.h](https://github.com/torvalds/linux/blob/master/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 with `mutexes`, let's consider the `mutex_waiter` structure. If you have read the [previous part](https://0xax.gitbooks.io/linux-insides/content/SyncPrim/sync-3.html) 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/master/kernel/locking/semaphore.c) source code file:
```C
struct semaphore_waiter {
@ -87,7 +87,7 @@ 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 are represent entry of the mutex wait queue. The one difference here that the `mutex_waiter` does not contain `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`.
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 a 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.gitbooks.io/linux-insides/content/SyncPrim/sync-4.html) of this chapter. From the theoretical side everything looks pretty simple. Let's look how `reader/writer semaphore` is represented in the Linux kernel.
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 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.gitbooks.io/linux-insides/content/SyncPrim/sync-4.html) of this chapter. From the theoretical side everything looks pretty simple. Let's look how `reader/writer semaphore` is represented in the Linux kernel.
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.gitbooks.io/linux-insides/content/SyncPrim/sync-5.html) 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 aqcuired 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.
Pavel V
8 years ago
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