一、概述

互斥锁struct mutex是内核访问临界区上锁的另一种方式，不同于自旋锁，在互斥锁上锁期间，另一个进程获取不到锁，可能会导致睡眠，直到前一个进程释放了互斥锁，后一个进程才会被唤醒，并获取该互斥锁。
由于互斥锁可能导致睡眠，所以不能在中断、tasklet和内核定时器中使用。

二、互斥锁

//in include/linux/mutex.h
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
};

struct mutex mutex_lock;

//初始化互斥锁
mutex_init(&mutex_lock);

//互斥锁加锁
mutex_lock(&mutex_lock);
mutex_lock_interruptible(&mutex_lock);
mutex_trylock(&mutex_lock);

//互斥锁解锁
mutex_unlock(&mutex_lock);

mutex_lock引起的睡眠不能被打断，而mutex_lock_interruptible可以被打断。mutex_trylock(&mutex_lock)是非阻塞的，若加锁成功返回1，失败返回0。
使用互斥锁的注意事项：

• 由于互斥锁可能导致睡眠，进而引起CPU调度，发生进程上下文的切换，所以消耗的资源是比较多的，只有当进程占用资源时间较长时，才选择互斥锁。反之可选择自旋锁
• 互斥锁不能在中断、软中断、tasklet和内核定时器等不允许睡眠的场景下使用(定时器处理函数是作为软中断在底半部执行的)
• 互斥锁无法递归加锁，无法重复解锁，这一点跟自旋锁也是一样

三、源码分析

#define MUTEX_FLAG_WAITERS	0x01
#define MUTEX_FLAG_HANDOFF	0x02
#define MUTEX_FLAG_PICKUP	0x04

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

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

static __always_inline bool __mutex_trylock_fast(struct mutex *lock)
{
	unsigned long curr = (unsigned long)current;

	if (!atomic_long_cmpxchg_acquire(&lock->owner, 0UL, curr))
		return true;

	return false;
}

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);
}

*
 * Lock a mutex (possibly interruptible), slowpath:
 */
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;
	}

	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_wakeup_for_backoff(lock, ww_ctx);

		goto skip_wait;
	}

	debug_mutex_lock_common(lock, &waiter);
	debug_mutex_add_waiter(lock, &waiter, current);

	lock_contended(&lock->dep_map, ip);

	if (!use_ww_ctx) {
		/* add waiting tasks to the end of the waitqueue (FIFO): */
		list_add_tail(&waiter.list, &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 back off. */
		ret = __ww_mutex_add_waiter(&waiter, lock, ww_ctx);
		if (ret)
			goto err_early_backoff;

		waiter.ww_ctx = ww_ctx;
	}

	waiter.task = current;

	if (__mutex_waiter_is_first(lock, &waiter))
		__mutex_set_flag(lock, MUTEX_FLAG_WAITERS);

	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 wound 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 && ww_ctx->acquired > 0) {
			ret = __ww_mutex_lock_check_stamp(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);

	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_set_context_slowpath(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_backoff:
	spin_unlock(&lock->wait_lock);
	debug_mutex_free_waiter(&waiter);
	mutex_release(&lock->dep_map, 1, ip);
	preempt_enable();
	return ret;
}

1. 首先mutex_lock调用__mutex_trylock_fast------>atomic_long_cmpxchg_acquire获取互斥锁，该函数将lock->owner和0UL进行比较，如果相等(该互斥锁没有被进程占用)，那么将第三个参数赋值给第一个参数，也就是lock->owner=curr，如果不相等，说明已经有进程加锁占用了，该函数什么都不执行，直接返回lock->owner。
2. 若步骤一加锁成功，则直接返回结果；如果步骤一加锁失败，继续执行__mutex_lock_slowpath尝试加锁，核心函数是__mutex_lock_common。
   __mutex_lock_slowpath----->
   __mutex_lock----------------->
   __mutex_lock_common—>
3. __mutex_lock_common里，首先执行preempt_disbale函数关闭抢占（只在加锁过程中关闭抢占），然后执行两个核心函数，其一是__mutex_trylock—>__mutex_trylock_or_owner，其二是mutex_optimistic_spin，前者尝试加锁，后者尝试自旋等待。
4. __mutex_trylock_or_owner中，取了lock->owner的持有者赋值给owner变量，owner变量由’struct task_struct * '指针，NULL 表示不拥有，以及低三位的标志状态组成。

• Bit0 MUTEX_FLAG_WAITERS，表示当前有锁的等待者
• Bit1 MUTEX_FLAG_HANDOFF，表示解锁需要将锁交给top-waiter
• Bit2 MUTEX_FLAG_PICKUP，表示已完成handoff标志位的确认。
  __mutex_trylock_or_owner中判断了当前进程和锁的持有者进程是否相等，以及当前锁持有者的bit2是否置位，如果条件不满足就会继续执行加锁函数。__mutex_trylock执行失败则继续执行第二个函数：mutex_optimistic_spin。

/*
 * Trylock variant that retuns the owning task on failure.
 */
static inline struct task_struct *__mutex_trylock_or_owner(struct mutex *lock)
{
	unsigned long owner, curr = (unsigned long)current;

	owner = atomic_long_read(&lock->owner);
	for (;;) { /* must loop, can race against a flag */
		unsigned long old, flags = __owner_flags(owner);
		unsigned long task = owner & ~MUTEX_FLAGS;

		if (task) {
			if (likely(task != curr))
				break;

			if (likely(!(flags & MUTEX_FLAG_PICKUP)))
				break;

			flags &= ~MUTEX_FLAG_PICKUP;
		} else {
#ifdef CONFIG_DEBUG_MUTEXES
			DEBUG_LOCKS_WARN_ON(flags & MUTEX_FLAG_PICKUP);
#endif
		}

		/*
		 * We set the HANDOFF bit, we must make sure it doesn't live
		 * past the point where we acquire it. This would be possible
		 * if we (accidentally) set the bit on an unlocked mutex.
		 */
		flags &= ~MUTEX_FLAG_HANDOFF;

		old = atomic_long_cmpxchg_acquire(&lock->owner, owner, curr | flags);
		if (old == owner)
			return NULL;

		owner = old;
	}

	return __owner_task(owner);
}

进入mutex_optimistic_spin函数时，waiter参数为NULL，该函数会先判断当前请求锁的进程能否进入到自旋等待状态，如果可以才会去执行osq_lock，并循环尝试加锁和自旋，若失败则返回进入睡眠状态

5. 如果以上尝试加锁和自旋都失败了，接下来就是进入睡眠
   将当前进程加入lock->wait_list等待队列，并设置bit0和bit1标志位，告知系统当前队列有等待者
   调用schedule_preempt_disabled进行系统调度
   当本进程被唤醒时，判断了如果是等待队列里的第一个，就设置handoff标志，标志着锁释放后，先给本进程加锁
   然后本进程调用__mutex_trylock加锁，如果加锁成功则跳出循环，加锁结束，将本进程从互斥锁等待队列移除等等，做一些清除工作。

No pains, no gains