Commit-ID: 7c34e3180a01c800a40bc8535654d5735802fc1b Gitweb: http://git.kernel.org/tip/7c34e3180a01c800a40bc8535654d5735802fc1b Author: Nicholas Mc Guire <der.herr@xxxxxxx> AuthorDate: Fri, 23 Jan 2015 12:41:47 +0100 Committer: Ingo Molnar <mingo@xxxxxxxxxx> CommitDate: Wed, 4 Feb 2015 07:57:37 +0100 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@xxxxxxx> Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx> Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> Link: http://lkml.kernel.org/r/1422013307-13200-1-git-send-email-der.herr@xxxxxxx Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx> --- kernel/sched/completion.c | 9 +++++++++ 1 file changed, 9 insertions(+) diff --git a/kernel/sched/completion.c b/kernel/sched/completion.c index 9d1fe32d..7052d3f 100644 --- a/kernel/sched/completion.c +++ b/kernel/sched/completion.c @@ -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; -- To unsubscribe from this list: send the line "unsubscribe linux-tip-commits" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
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