From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755195AbcIEHPE (ORCPT ); Mon, 5 Sep 2016 03:15:04 -0400 Received: from gate.crashing.org ([63.228.1.57]:57893 "EHLO gate.crashing.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754356AbcIEHPC (ORCPT ); Mon, 5 Sep 2016 03:15:02 -0400 Message-ID: <1473059659.2313.45.camel@kernel.crashing.org> Subject: Re: [RESEND][v2][PATCH] Fix a race between try_to_wake_up() and a woken up task From: Benjamin Herrenschmidt To: Balbir Singh , Peter Zijlstra , LKML , Oleg Nesterov Cc: Nicholas Piggin , Alexey Kardashevskiy Date: Mon, 05 Sep 2016 17:14:19 +1000 In-Reply-To: References: <33872563-f4bb-0b01-6848-aaa8482aa91f@gmail.com> Content-Type: text/plain; charset="UTF-8" X-Mailer: Evolution 3.20.5 (3.20.5-1.fc24) Mime-Version: 1.0 Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, 2016-09-05 at 13:16 +1000, Balbir Singh wrote:  .../... > > Cc: Peter Zijlstra > Cc: Nicholas Piggin Acked-by: Benjamin Herrenschmidt > Signed-off-by: Balbir Singh > --- >  kernel/sched/core.c | 11 +++++++++++ >  1 file changed, 11 insertions(+) > > diff --git a/kernel/sched/core.c b/kernel/sched/core.c > index 2a906f2..582c684 100644 > --- a/kernel/sched/core.c > +++ b/kernel/sched/core.c > @@ -2016,6 +2016,17 @@ try_to_wake_up(struct task_struct *p, unsigned > int state, int wake_flags) >   success = 1; /* we're going to change ->state */ >   cpu = task_cpu(p); >   > + /* > +  * Ensure we see on_rq and p_state consistently > +  * > +  * For example in __rwsem_down_write_failed(), we have > +  *    [S] ->on_rq = 1 [L] > ->state > +  *    MB  RMB > +  *    [S] ->state = TASK_UNINTERRUPTIBLE [L] ->on_rq > +  * In the absence of the RMB p->on_rq can be observed to be > 0 > +  * and we end up spinning indefinitely in while (p->on_cpu) > +  */ > + smp_rmb(); >   if (p->on_rq && ttwu_remote(p, wake_flags)) >   goto stat; >