Linux 内核同步（六）：互斥体（mutex）

互斥体

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互斥体是一种睡眠锁，他是一种简单的睡眠锁，其行为和 count 为 1 的信号量类似。（关于信号量参考：Linux 内核同步（四）：信号量 semaphore）。

互斥体简洁高效，但是相比信号量，有更多的限制，因此对于互斥体的使用条件更加严格：

• 任何时刻，只有一个指定的任务允许持有 mutex，也就是说，mutex 的计数永远是 1；
• 给 mutex 上锁这，必须负责给他解锁，也就是不允许在一个上下文中上锁，在另外一个上下文中解锁。这个限制注定了 mutex 无法承担内核和用户空间同步的复杂场景。常用的方式是在一个上下文中进行上锁/解锁。
• 递归的调用上锁和解锁是不允许的。也就是说，不能递归的去持有同一个锁，也不能够递归的解开一个已经解开的锁。
• 当持有 mutex 的进程，不允许退出
• mutex 不允许在中断上下文和软中断上下文中使用过，即便是mutex_trylock 也不行
• mutex 只能使用内核提供的 APIs操作，不允许拷贝，手动初始化和重复初始化
•  

信号量和互斥体

他们两者很相似，除非是 mutex 的限制妨碍到逻辑，否则这两者之间，首选 mutex

 

自旋锁和互斥体

多数情况，很好区分。中断中只能考虑自旋锁，任务睡眠使用互斥体。如果都可以的的情况下，低开销或者短时间的锁，选择自旋锁，长期加锁的话，使用互斥体。

 

互斥体的使用

函数定义	功能说明
mutex_lock(struct mutex *lock)	加锁，如果不可用，则睡眠（UNINTERRUPTIBLE）
mutex_lock_interruptible(struct mutex *lock);	加锁，如果不可用，则睡眠（TASK_INTERRUPTIBLE）
mutex_unlock(struct mutex *lock)	解锁
mutex_trylock(struct mutex *lock)	试图获取指定的 mutex，或得到返回1，否则返回 0
mutex_is_locked(struct mutex *lock)	如果 mutex 被占用返回1，否则返回 0

 

互斥体的实现

互斥体的定义在：include/linux/mutex.h

1. mutex 的结构

struct mutex {
	atomic_long_t		owner;
	spinlock_t		wait_lock;
#ifdef CONFIG_MUTEX_SPIN_ON_OWNER
	struct optimistic_spin_queue osq; /* Spinner MCS lock */
#endif
	struct list_head	wait_list;
#ifdef CONFIG_DEBUG_MUTEXES
	void			*magic;
#endif
#ifdef CONFIG_DEBUG_LOCK_ALLOC
	struct lockdep_map	dep_map;
#endif
};

 

2. mutex 初始化

void
__mutex_init(struct mutex *lock, const char *name, struct lock_class_key *key)
{
	atomic_long_set(&lock->owner, 0);
	spin_lock_init(&lock->wait_lock);
	INIT_LIST_HEAD(&lock->wait_list);
#ifdef CONFIG_MUTEX_SPIN_ON_OWNER
	osq_lock_init(&lock->osq);
#endif

	debug_mutex_init(lock, name, key);
}
EXPORT_SYMBOL(__mutex_init);

 

3. mutex 加锁

void __sched mutex_lock(struct mutex *lock)
{
	might_sleep();

	if (!__mutex_trylock_fast(lock))
		__mutex_lock_slowpath(lock);
}

首先 check 是否能够获得锁，否则调用到 __mutex_lock_slowpath：

static noinline void __sched
__mutex_lock_slowpath(struct mutex *lock)
{
	__mutex_lock(lock, TASK_UNINTERRUPTIBLE, 0, NULL, _RET_IP_);
}

static int __sched
__mutex_lock(struct mutex *lock, long state, unsigned int subclass,
	     struct lockdep_map *nest_lock, unsigned long ip)
{
	return __mutex_lock_common(lock, state, subclass, nest_lock, ip, NULL, false);
}

所以调用到了 __mutex_lock_common 函数：

static __always_inline int __sched
__mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
		    struct lockdep_map *nest_lock, unsigned long ip,
		    struct ww_acquire_ctx *ww_ctx, const bool use_ww_ctx)
{
	struct mutex_waiter waiter;
	bool first = false;
	struct ww_mutex *ww;
	int ret;

	might_sleep();

	ww = container_of(lock, struct ww_mutex, base);
	if (use_ww_ctx && ww_ctx) {
		if (unlikely(ww_ctx == READ_ONCE(ww->ctx)))
			return -EALREADY;

		/*
		 * Reset the wounded flag after a kill. No other process can
		 * race and wound us here since they can't have a valid owner
		 * pointer if we don't have any locks held.
		 */
		if (ww_ctx->acquired == 0)
			ww_ctx->wounded = 0;
	}

	preempt_disable();
	mutex_acquire_nest(&lock->dep_map, subclass, 0, nest_lock, ip);

	if (__mutex_trylock(lock) ||
	    mutex_optimistic_spin(lock, ww_ctx, use_ww_ctx, NULL)) {
		/* got the lock, yay! */
		lock_acquired(&lock->dep_map, ip);
		if (use_ww_ctx && ww_ctx)
			ww_mutex_set_context_fastpath(ww, ww_ctx);
		preempt_enable();
		return 0;
	}

	spin_lock(&lock->wait_lock);
	/*
	 * After waiting to acquire the wait_lock, try again.
	 */
	if (__mutex_trylock(lock)) {
		if (use_ww_ctx && ww_ctx)
			__ww_mutex_check_waiters(lock, ww_ctx);

		goto skip_wait;
	}

	debug_mutex_lock_common(lock, &waiter);

	lock_contended(&lock->dep_map, ip);

	if (!use_ww_ctx) {
		/* add waiting tasks to the end of the waitqueue (FIFO): */
		__mutex_add_waiter(lock, &waiter, &lock->wait_list);


#ifdef CONFIG_DEBUG_MUTEXES
		waiter.ww_ctx = MUTEX_POISON_WW_CTX;
#endif
	} else {
		/*
		 * Add in stamp order, waking up waiters that must kill
		 * themselves.
		 */
		ret = __ww_mutex_add_waiter(&waiter, lock, ww_ctx);
		if (ret)
			goto err_early_kill;

		waiter.ww_ctx = ww_ctx;
	}

	waiter.task = current;

	set_current_state(state);
	for (;;) {
		/*
		 * Once we hold wait_lock, we're serialized against
		 * mutex_unlock() handing the lock off to us, do a trylock
		 * before testing the error conditions to make sure we pick up
		 * the handoff.
		 */
		if (__mutex_trylock(lock))
			goto acquired;

		/*
		 * Check for signals and kill conditions while holding
		 * wait_lock. This ensures the lock cancellation is ordered
		 * against mutex_unlock() and wake-ups do not go missing.
		 */
		if (unlikely(signal_pending_state(state, current))) {
			ret = -EINTR;
			goto err;
		}

		if (use_ww_ctx && ww_ctx) {
			ret = __ww_mutex_check_kill(lock, &waiter, ww_ctx);
			if (ret)
				goto err;
		}

		spin_unlock(&lock->wait_lock);
		schedule_preempt_disabled();

		/*
		 * ww_mutex needs to always recheck its position since its waiter
		 * list is not FIFO ordered.
		 */
		if ((use_ww_ctx && ww_ctx) || !first) {
			first = __mutex_waiter_is_first(lock, &waiter);
			if (first)
				__mutex_set_flag(lock, MUTEX_FLAG_HANDOFF);
		}

		set_current_state(state);
		/*
		 * Here we order against unlock; we must either see it change
		 * state back to RUNNING and fall through the next schedule(),
		 * or we must see its unlock and acquire.
		 */
		if (__mutex_trylock(lock) ||
		    (first && mutex_optimistic_spin(lock, ww_ctx, use_ww_ctx, &waiter)))
			break;

		spin_lock(&lock->wait_lock);
	}
	spin_lock(&lock->wait_lock);
acquired:
	__set_current_state(TASK_RUNNING);

	if (use_ww_ctx && ww_ctx) {
		/*
		 * Wound-Wait; we stole the lock (!first_waiter), check the
		 * waiters as anyone might want to wound us.
		 */
		if (!ww_ctx->is_wait_die &&
		    !__mutex_waiter_is_first(lock, &waiter))
			__ww_mutex_check_waiters(lock, ww_ctx);
	}

	mutex_remove_waiter(lock, &waiter, current);
	if (likely(list_empty(&lock->wait_list)))
		__mutex_clear_flag(lock, MUTEX_FLAGS);

	debug_mutex_free_waiter(&waiter);

skip_wait:
	/* got the lock - cleanup and rejoice! */
	lock_acquired(&lock->dep_map, ip);

	if (use_ww_ctx && ww_ctx)
		ww_mutex_lock_acquired(ww, ww_ctx);

	spin_unlock(&lock->wait_lock);
	preempt_enable();
	return 0;

err:
	__set_current_state(TASK_RUNNING);
	mutex_remove_waiter(lock, &waiter, current);
err_early_kill:
	spin_unlock(&lock->wait_lock);
	debug_mutex_free_waiter(&waiter);
	mutex_release(&lock->dep_map, 1, ip);
	preempt_enable();
	return ret;
}

进入等待队列。

 

4. mutex 解锁

void __sched mutex_unlock(struct mutex *lock)
{
#ifndef CONFIG_DEBUG_LOCK_ALLOC
	if (__mutex_unlock_fast(lock))
		return;
#endif
	__mutex_unlock_slowpath(lock, _RET_IP_);
}
EXPORT_SYMBOL(mutex_unlock);

调用到 __mutex_unlock_slowpath ：

static noinline void __sched __mutex_unlock_slowpath(struct mutex *lock, unsigned long ip)
{
	struct task_struct *next = NULL;
	DEFINE_WAKE_Q(wake_q);
	unsigned long owner;

	mutex_release(&lock->dep_map, 1, ip);

	/*
	 * Release the lock before (potentially) taking the spinlock such that
	 * other contenders can get on with things ASAP.
	 *
	 * Except when HANDOFF, in that case we must not clear the owner field,
	 * but instead set it to the top waiter.
	 */
	owner = atomic_long_read(&lock->owner);
	for (;;) {
		unsigned long old;

#ifdef CONFIG_DEBUG_MUTEXES
		DEBUG_LOCKS_WARN_ON(__owner_task(owner) != current);
		DEBUG_LOCKS_WARN_ON(owner & MUTEX_FLAG_PICKUP);
#endif

		if (owner & MUTEX_FLAG_HANDOFF)
			break;

		old = atomic_long_cmpxchg_release(&lock->owner, owner,
						  __owner_flags(owner));
		if (old == owner) {
			if (owner & MUTEX_FLAG_WAITERS)
				break;

			return;
		}

		owner = old;
	}

	spin_lock(&lock->wait_lock);
	debug_mutex_unlock(lock);
	if (!list_empty(&lock->wait_list)) {
		/* get the first entry from the wait-list: */
		struct mutex_waiter *waiter =
			list_first_entry(&lock->wait_list,
					 struct mutex_waiter, list);

		next = waiter->task;

		debug_mutex_wake_waiter(lock, waiter);
		wake_q_add(&wake_q, next);
	}

	if (owner & MUTEX_FLAG_HANDOFF)
		__mutex_handoff(lock, next);

	spin_unlock(&lock->wait_lock);

	wake_up_q(&wake_q);
}

做唤醒操作。