Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751778AbdIAJXc (ORCPT ); Fri, 1 Sep 2017 05:23:32 -0400 Received: from mail-wm0-f67.google.com ([74.125.82.67]:37297 "EHLO mail-wm0-f67.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751559AbdIAJXa (ORCPT ); Fri, 1 Sep 2017 05:23:30 -0400 X-Google-Smtp-Source: ADKCNb4XQiecmgwbjlOmCDYmTxi4V88Gkh8RLK6jzbVD/UbCUyh4bshH1IYzC3PwgbarOo94pdkDMA== Date: Fri, 1 Sep 2017 11:23:22 +0200 From: Andrea Parri To: Nicholas Piggin Cc: Ingo Molnar , Peter Zijlstra , Paul McKenney , Linus Torvalds , linux-kernel@vger.kernel.org Subject: Re: [PATCH] swait: add missing barrier to swake_up Message-ID: <20170901092322.GA4192@andrea> References: <20170901061450.1450-1-npiggin@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20170901061450.1450-1-npiggin@gmail.com> User-Agent: Mutt/1.5.24 (2015-08-30) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1961 Lines: 67 On Fri, Sep 01, 2017 at 04:14:50PM +1000, Nicholas Piggin wrote: > 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. Hi Nicholas. I noticed 35a2897c2a306cca344ca5c0b43416707018f434 ("sched/wait: Remove the lockless swait_active() check in swake_up*()") in tip:locking/core. Andrea > > 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 >