Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S965592AbeAOMjf (ORCPT + 1 other); Mon, 15 Jan 2018 07:39:35 -0500 Received: from mail.linuxfoundation.org ([140.211.169.12]:46312 "EHLO mail.linuxfoundation.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S965533AbeAOMjY (ORCPT ); Mon, 15 Jan 2018 07:39:24 -0500 From: Greg Kroah-Hartman To: linux-kernel@vger.kernel.org Cc: Greg Kroah-Hartman , stable@vger.kernel.org, Jianyu Zhan , Christian Borntraeger , Darren Hart , dave@stgolabs.net, peterz@infradead.org, linux@rasmusvillemoes.dk, akpm@linux-foundation.org, fengguang.wu@intel.com, bigeasy@linutronix.de, Thomas Gleixner , Davidlohr Bueso Subject: [PATCH 4.4 31/87] futex: Replace barrier() in unqueue_me() with READ_ONCE() Date: Mon, 15 Jan 2018 13:34:30 +0100 Message-Id: <20180115123352.472950416@linuxfoundation.org> X-Mailer: git-send-email 2.15.1 In-Reply-To: <20180115123349.252309699@linuxfoundation.org> References: <20180115123349.252309699@linuxfoundation.org> User-Agent: quilt/0.65 MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Return-Path: 4.4-stable review patch. If anyone has any objections, please let me know. ------------------ From: Jianyu Zhan commit 29b75eb2d56a714190a93d7be4525e617591077a upstream. 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 Signed-off-by: Davidlohr Bueso Signed-off-by: Greg Kroah-Hartman --- kernel/futex.c | 8 ++++++-- 1 file changed, 6 insertions(+), 2 deletions(-) --- a/kernel/futex.c +++ b/kernel/futex.c @@ -1939,8 +1939,12 @@ 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); /*