Commit 7c34e318 authored Jan 23, 2015 by Nicholas Mc Guire Committed by Ingo Molnar Feb 04, 2015

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: Nicholas Mc Guire <der.herr@hofr.at>
Signed-off-by: Peter 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.at


Signed-off-by: Ingo Molnar <mingo@kernel.org>

parent de30ec47

kernel/sched/completion.c

+9 −0

Original line number	Diff line number	Diff line
		@@ -268,6 +268,15 @@ bool try_wait_for_completion(struct completion *x)
		unsigned long flags;
		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);
		if (!x->done)
		ret = 0;