Commit 7c34e318 authored by Nicholas Mc Guire's avatar Nicholas Mc Guire Committed by Ingo Molnar

sched/completion: Add lock-free checking of the blocking case

The "thread would block" case can be checked without grabbing ->wait.lock.

[ If the check does not return early then grab the lock and recheck.
  A memory barrier is not needed as complete() and complete_all() imply
  a barrier.

  The ACCESS_ONCE() is needed for calls in a loop that, if inlined, could
  optimize out the re-fetching of x->done. ]
Signed-off-by: default avatarNicholas Mc Guire <der.herr@hofr.at>
Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: http://lkml.kernel.org/r/1422013307-13200-1-git-send-email-der.herr@hofr.atSigned-off-by: default avatarIngo Molnar <mingo@kernel.org>
parent de30ec47
...@@ -268,6 +268,15 @@ bool try_wait_for_completion(struct completion *x) ...@@ -268,6 +268,15 @@ bool try_wait_for_completion(struct completion *x)
unsigned long flags; unsigned long flags;
int ret = 1; int ret = 1;
/*
* Since x->done will need to be locked only
* in the non-blocking case, we check x->done
* first without taking the lock so we can
* return early in the blocking case.
*/
if (!ACCESS_ONCE(x->done))
return 0;
spin_lock_irqsave(&x->wait.lock, flags); spin_lock_irqsave(&x->wait.lock, flags);
if (!x->done) if (!x->done)
ret = 0; ret = 0;
......
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