sched/wait: Add swq_has_sleeper()
Which is the equivalent of what we have in regular waitqueues. I'm not crazy about the name, but this also helps us get both apis closer -- which iirc comes originally from the -net folks. We also duplicate the comments for the lockless swait_active(), from wait.h. Future users will make use of this interface. Signed-off-by: Davidlohr Bueso <dbueso@suse.de> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
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@ -79,9 +79,63 @@ extern void __init_swait_queue_head(struct swait_queue_head *q, const char *name
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DECLARE_SWAIT_QUEUE_HEAD(name)
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#endif
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static inline int swait_active(struct swait_queue_head *q)
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/**
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* swait_active -- locklessly test for waiters on the queue
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* @wq: the waitqueue to test for waiters
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*
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* returns true if the wait list is not empty
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*
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* NOTE: this function is lockless and requires care, incorrect usage _will_
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* lead to sporadic and non-obvious failure.
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*
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* NOTE2: this function has the same above implications as regular waitqueues.
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*
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* Use either while holding swait_queue_head::lock or when used for wakeups
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* with an extra smp_mb() like:
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*
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* CPU0 - waker CPU1 - waiter
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*
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* for (;;) {
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* @cond = true; prepare_to_swait(&wq_head, &wait, state);
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* smp_mb(); // smp_mb() from set_current_state()
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* if (swait_active(wq_head)) if (@cond)
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* wake_up(wq_head); break;
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* schedule();
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* }
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* finish_swait(&wq_head, &wait);
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*
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* Because without the explicit smp_mb() it's possible for the
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* swait_active() load to get hoisted over the @cond store such that we'll
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* observe an empty wait list while the waiter might not observe @cond.
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* This, in turn, can trigger missing wakeups.
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*
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* Also note that this 'optimization' trades a spin_lock() for an smp_mb(),
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* which (when the lock is uncontended) are of roughly equal cost.
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*/
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static inline int swait_active(struct swait_queue_head *wq)
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{
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return !list_empty(&q->task_list);
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return !list_empty(&wq->task_list);
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}
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/**
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* swq_has_sleeper - check if there are any waiting processes
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* @wq: the waitqueue to test for waiters
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*
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* Returns true if @wq has waiting processes
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*
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* Please refer to the comment for swait_active.
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*/
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static inline bool swq_has_sleeper(struct swait_queue_head *wq)
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{
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/*
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* We need to be sure we are in sync with the list_add()
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* modifications to the wait queue (task_list).
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*
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* This memory barrier should be paired with one on the
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* waiting side.
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*/
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smp_mb();
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return swait_active(wq);
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}
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extern void swake_up(struct swait_queue_head *q);
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