linux-kernel - [PATCH RFC v3] sched: completion: locks should not be needed for reading completion state

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Message-Id: <1422030058-13611-1-git-send-email-der.herr@hofr.at>
Date:	Fri, 23 Jan 2015 17:20:58 +0100
From:	Nicholas Mc Guire <der.herr@...r.at>
To:	Ingo Molnar <mingo@...hat.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel@...r.kernel.org, Nicholas Mc Guire <der.herr@...r.at>
Subject: [PATCH RFC v3] sched: completion: locks should not be needed for reading completion state

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

v2: proper subsystem tag in subject line
v3: typo in return fixed

Sorry for this resend - just noticed that I had - carelessly lost the
second inversion somehow - logic should be correct now.

The ACCESS_ONCE is needed for calls in a loop that, if inlined, could
optimize out the re-fetching of x->done. An explicit memory barrier is 
not needed as complete() and complete_all() imply a barrier.

 kernel/sched/completion.c |    9 +--------
 1 file changed, 1 insertion(+), 8 deletions(-)

diff --git a/kernel/sched/completion.c b/kernel/sched/completion.c
index 607f852..7c5cd70 100644
--- a/kernel/sched/completion.c
+++ b/kernel/sched/completion.c
@@ -288,13 +288,6 @@ EXPORT_SYMBOL(try_wait_for_completion);
  */
 bool completion_done(struct completion *x)
 {
-	unsigned long flags;
-	int ret = 1;
-
-	spin_lock_irqsave(&x->wait.lock, flags);
-	if (!x->done)
-		ret = 0;
-	spin_unlock_irqrestore(&x->wait.lock, flags);
-	return ret;
+	return !!ACCESS_ONCE(x->done);
 }
 EXPORT_SYMBOL(completion_done);
-- 
1.7.10.4

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