Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751330AbdIAGPH (ORCPT ); Fri, 1 Sep 2017 02:15:07 -0400 Received: from mail-pg0-f65.google.com ([74.125.83.65]:34758 "EHLO mail-pg0-f65.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750981AbdIAGPF (ORCPT ); Fri, 1 Sep 2017 02:15:05 -0400 X-Google-Smtp-Source: ADKCNb5Sqq7uShpn9M2U9eODb3c6VvziaW4JUWo0UfMkH6/HFwx78nn7h7Xzb8eKRF3V3xtselUd2g== From: Nicholas Piggin To: Ingo Molnar Cc: Nicholas Piggin , Peter Zijlstra , Paul McKenney , Linus Torvalds , linux-kernel@vger.kernel.org Subject: [PATCH] swait: add missing barrier to swake_up Date: Fri, 1 Sep 2017 16:14:50 +1000 Message-Id: <20170901061450.1450-1-npiggin@gmail.com> X-Mailer: git-send-email 2.13.3 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1609 Lines: 54 swake_up and swake_up_all test the swaitqueue outside the lock, but they are missing the barrier that would ensure visibility of a previous store that sets the wakeup condition with the load that tests the swaitqueue. This could lead to a lost wakeup if there is memory reordering. Fix this as prescribed by the waitqueue_active comments. Signed-off-by: Nicholas Piggin -- I noticed this when chasing down that rcu hang bug (which turned out to not be anything of the sort). I might be missing something here and it's safe somehow, but if so then it should have a comment where it diverges from normal waitqueues. It looks like there's a few callers which are also testing swait_active before swake_up without a barrier which look wrong, so I must be missing something but I'm not sure what. Thanks, Nick --- kernel/sched/swait.c | 10 ++++++++++ 1 file changed, 10 insertions(+) diff --git a/kernel/sched/swait.c b/kernel/sched/swait.c index 3d5610dcce11..9056278001d9 100644 --- a/kernel/sched/swait.c +++ b/kernel/sched/swait.c @@ -33,6 +33,11 @@ void swake_up(struct swait_queue_head *q) { unsigned long flags; + /* + * See waitqueue_active() comments for checking waiters outside + * the lock. Same principle applies here. + */ + smp_mb(); if (!swait_active(q)) return; @@ -51,6 +56,11 @@ void swake_up_all(struct swait_queue_head *q) struct swait_queue *curr; LIST_HEAD(tmp); + /* + * See waitqueue_active() comments for checking waiters outside + * the lock. Same principle applies here. + */ + smp_mb(); if (!swait_active(q)) return; -- 2.13.3