[tip:locking/core] sched/completion: Add lock-free checking of the blocking case

[tip-bot for Nicholas Mc Guire]

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;
--
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