From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S966328AbbBDOik (ORCPT ); Wed, 4 Feb 2015 09:38:40 -0500 Received: from terminus.zytor.com ([198.137.202.10]:38825 "EHLO terminus.zytor.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S966224AbbBDOih (ORCPT ); Wed, 4 Feb 2015 09:38:37 -0500 Date: Wed, 4 Feb 2015 06:38:04 -0800 From: tip-bot for Nicholas Mc Guire Message-ID: Cc: der.herr@hofr.at, peterz@infradead.org, hpa@zytor.com, tglx@linutronix.de, mingo@kernel.org, torvalds@linux-foundation.org, linux-kernel@vger.kernel.org Reply-To: linux-kernel@vger.kernel.org, torvalds@linux-foundation.org, mingo@kernel.org, tglx@linutronix.de, hpa@zytor.com, peterz@infradead.org, der.herr@hofr.at In-Reply-To: <1422013307-13200-1-git-send-email-der.herr@hofr.at> References: <1422013307-13200-1-git-send-email-der.herr@hofr.at> To: linux-tip-commits@vger.kernel.org Subject: [tip:locking/core] sched/completion: Add lock-free checking of the blocking case Git-Commit-ID: 7c34e3180a01c800a40bc8535654d5735802fc1b X-Mailer: tip-git-log-daemon Robot-ID: Robot-Unsubscribe: Contact to get blacklisted from these emails MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Content-Type: text/plain; charset=UTF-8 Content-Disposition: inline Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Commit-ID: 7c34e3180a01c800a40bc8535654d5735802fc1b Gitweb: http://git.kernel.org/tip/7c34e3180a01c800a40bc8535654d5735802fc1b Author: Nicholas Mc Guire AuthorDate: Fri, 23 Jan 2015 12:41:47 +0100 Committer: Ingo Molnar 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 Signed-off-by: Peter Zijlstra (Intel) Cc: Linus Torvalds Link: http://lkml.kernel.org/r/1422013307-13200-1-git-send-email-der.herr@hofr.at Signed-off-by: Ingo Molnar --- 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;