From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752380AbaHMIYx (ORCPT ); Wed, 13 Aug 2014 04:24:53 -0400 Received: from terminus.zytor.com ([198.137.202.10]:52050 "EHLO terminus.zytor.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752104AbaHMIYt (ORCPT ); Wed, 13 Aug 2014 04:24:49 -0400 Date: Wed, 13 Aug 2014 01:23:54 -0700 From: tip-bot for Sebastian Andrzej Siewior Message-ID: Cc: linux-kernel@vger.kernel.org, hpa@zytor.com, mingo@kernel.org, torvalds@linux-foundation.org, peterz@infradead.org, acme@kernel.org, tglx@linutronix.de, bigeasy@linutronix.de Reply-To: mingo@kernel.org, hpa@zytor.com, linux-kernel@vger.kernel.org, torvalds@linux-foundation.org, peterz@infradead.org, acme@kernel.org, tglx@linutronix.de, bigeasy@linutronix.de In-Reply-To: <1407159068-1478-1-git-send-email-bigeasy@linutronix.de> References: <1407159068-1478-1-git-send-email-bigeasy@linutronix.de> To: linux-tip-commits@vger.kernel.org Subject: [tip:perf/core] perf: Do poll_wait() before checking condition in perf_poll() Git-Commit-ID: e708d7ad80737496870fd0b6794704d063fb0cdc 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: e708d7ad80737496870fd0b6794704d063fb0cdc Gitweb: http://git.kernel.org/tip/e708d7ad80737496870fd0b6794704d063fb0cdc Author: Sebastian Andrzej Siewior AuthorDate: Mon, 4 Aug 2014 15:31:08 +0200 Committer: Ingo Molnar CommitDate: Wed, 13 Aug 2014 07:51:11 +0200 perf: Do poll_wait() before checking condition in perf_poll() One should first enqueue to the waitqueue and then check for the condition. If the condition gets true after mutex_unlock() but before poll_wait() then we lose it and would have wait for another wakeup. This has been like this since v2.6.31-rc1 commit c7138f37f9 ("perf_counter: fix perf_poll()"). Before that it was slightly worse. I guess we get enough wakeups so if we miss here one it doesn't really matter. It is still a bad example. Signed-off-by: Sebastian Andrzej Siewior Signed-off-by: Peter Zijlstra Link: http://lkml.kernel.org/r/1407159068-1478-1-git-send-email-bigeasy@linutronix.de Cc: Arnaldo Carvalho de Melo Cc: Linus Torvalds Signed-off-by: Ingo Molnar --- kernel/events/core.c | 4 +--- 1 file changed, 1 insertion(+), 3 deletions(-) diff --git a/kernel/events/core.c b/kernel/events/core.c index a254605..2d7363a 100644 --- a/kernel/events/core.c +++ b/kernel/events/core.c @@ -3629,6 +3629,7 @@ static unsigned int perf_poll(struct file *file, poll_table *wait) struct ring_buffer *rb; unsigned int events = POLL_HUP; + poll_wait(file, &event->waitq, wait); /* * Pin the event->rb by taking event->mmap_mutex; otherwise * perf_event_set_output() can swizzle our rb and make us miss wakeups. @@ -3638,9 +3639,6 @@ static unsigned int perf_poll(struct file *file, poll_table *wait) if (rb) events = atomic_xchg(&rb->poll, 0); mutex_unlock(&event->mmap_mutex); - - poll_wait(file, &event->waitq, wait); - return events; }