Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932790AbcCHQKJ (ORCPT ); Tue, 8 Mar 2016 11:10:09 -0500 Received: from torg.zytor.com ([198.137.202.12]:58370 "EHLO terminus.zytor.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1751787AbcCHQJ6 (ORCPT ); Tue, 8 Mar 2016 11:09:58 -0500 Date: Tue, 8 Mar 2016 08:09:39 -0800 From: tip-bot for Jianyu Zhan Message-ID: Cc: linux-kernel@vger.kernel.org, nasa4836@gmail.com, borntraeger@de.ibm.com, tglx@linutronix.de, hpa@zytor.com, dvhart@linux.intel.com, mingo@kernel.org Reply-To: dvhart@linux.intel.com, mingo@kernel.org, hpa@zytor.com, tglx@linutronix.de, nasa4836@gmail.com, borntraeger@de.ibm.com, linux-kernel@vger.kernel.org In-Reply-To: <1457314344-5685-1-git-send-email-nasa4836@gmail.com> References: <1457314344-5685-1-git-send-email-nasa4836@gmail.com> To: linux-tip-commits@vger.kernel.org Subject: [tip:locking/core] futex: Replace barrier() in unqueue_me() with READ_ONCE() Git-Commit-ID: 29b75eb2d56a714190a93d7be4525e617591077a 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 Content-Length: 2177 Lines: 60 Commit-ID: 29b75eb2d56a714190a93d7be4525e617591077a Gitweb: http://git.kernel.org/tip/29b75eb2d56a714190a93d7be4525e617591077a Author: Jianyu Zhan AuthorDate: Mon, 7 Mar 2016 09:32:24 +0800 Committer: Thomas Gleixner CommitDate: Tue, 8 Mar 2016 17:04:02 +0100 futex: Replace barrier() in unqueue_me() with READ_ONCE() Commit e91467ecd1ef ("bug in futex unqueue_me") introduced a barrier() in unqueue_me() to prevent the compiler from rereading the lock pointer which might change after a check for NULL. Replace the barrier() with a READ_ONCE() for the following reasons: 1) READ_ONCE() is a weaker form of barrier() that affects only the specific load operation, while barrier() is a general compiler level memory barrier. READ_ONCE() was not available at the time when the barrier was added. 2) Aside of that READ_ONCE() is descriptive and self explainatory while a barrier without comment is not clear to the casual reader. No functional change. [ tglx: Massaged changelog ] Signed-off-by: Jianyu Zhan Acked-by: Christian Borntraeger Acked-by: Darren Hart Cc: dave@stgolabs.net Cc: peterz@infradead.org Cc: linux@rasmusvillemoes.dk Cc: akpm@linux-foundation.org Cc: fengguang.wu@intel.com Cc: bigeasy@linutronix.de Link: http://lkml.kernel.org/r/1457314344-5685-1-git-send-email-nasa4836@gmail.com Signed-off-by: Thomas Gleixner --- kernel/futex.c | 8 ++++++-- 1 file changed, 6 insertions(+), 2 deletions(-) diff --git a/kernel/futex.c b/kernel/futex.c index bae542e..a5d2e74 100644 --- a/kernel/futex.c +++ b/kernel/futex.c @@ -2010,8 +2010,12 @@ static int unqueue_me(struct futex_q *q) /* In the common case we don't take the spinlock, which is nice. */ retry: - lock_ptr = q->lock_ptr; - barrier(); + /* + * q->lock_ptr can change between this read and the following spin_lock. + * Use READ_ONCE to forbid the compiler from reloading q->lock_ptr and + * optimizing lock_ptr out of the logic below. + */ + lock_ptr = READ_ONCE(q->lock_ptr); if (lock_ptr != NULL) { spin_lock(lock_ptr); /*