sched/wait: Add swq_has_sleeper()

	DECLARE_SWAIT_QUEUE_HEAD(name)

	DECLARE_SWAIT_QUEUE_HEAD(name)

#endif

#endif

static inline int swait_active(struct swait_queue_head *q)

/**

 * swait_active -- locklessly test for waiters on the queue

 * @wq: the waitqueue to test for waiters

 *

 * returns true if the wait list is not empty

 *

 * NOTE: this function is lockless and requires care, incorrect usage _will_

 * lead to sporadic and non-obvious failure.

 *

 * NOTE2: this function has the same above implications as regular waitqueues.

 *

 * Use either while holding swait_queue_head::lock or when used for wakeups

 * with an extra smp_mb() like:

 *

 *      CPU0 - waker                    CPU1 - waiter

 *

 *                                      for (;;) {

 *      @cond = true;                     prepare_to_swait(&wq_head, &wait, state);

 *      smp_mb();                         // smp_mb() from set_current_state()

 *      if (swait_active(wq_head))        if (@cond)

 *        wake_up(wq_head);                      break;

 *                                        schedule();

 *                                      }

 *                                      finish_swait(&wq_head, &wait);

 *

 * Because without the explicit smp_mb() it's possible for the

 * swait_active() load to get hoisted over the @cond store such that we'll

 * observe an empty wait list while the waiter might not observe @cond.

 * This, in turn, can trigger missing wakeups.

 *

 * Also note that this 'optimization' trades a spin_lock() for an smp_mb(),

 * which (when the lock is uncontended) are of roughly equal cost.

 */

static inline int swait_active(struct swait_queue_head *wq)

{

{

	return !list_empty(&q->task_list);

	return !list_empty(&wq->task_list);

}

/**

 * swq_has_sleeper - check if there are any waiting processes

 * @wq: the waitqueue to test for waiters

 *

 * Returns true if @wq has waiting processes

 *

 * Please refer to the comment for swait_active.

 */

static inline bool swq_has_sleeper(struct swait_queue_head *wq)

{

	/*

	 * We need to be sure we are in sync with the list_add()

	 * modifications to the wait queue (task_list).

	 *

	 * This memory barrier should be paired with one on the

	 * waiting side.

	 */

	smp_mb();

	return swait_active(wq);

}

}

extern void swake_up(struct swait_queue_head *q);

extern void swake_up(struct swait_queue_head *q);