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sched/wait: Add swq_has_sleeper()

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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>
This commit is contained in:
Davidlohr Bueso 2017-09-13 13:08:18 -07:00 committed by Paolo Bonzini
parent 3a8b0677fc
commit 8cd641e3c7
1 changed files with 56 additions and 2 deletions
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58
include/linux/swait.h
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@ -79,9 +79,63 @@ extern void __init_swait_queue_head(struct swait_queue_head *q, const char *name
DECLARE_SWAIT_QUEUE_HEAD(name)
#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);