From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756294AbZBKKEq (ORCPT ); Wed, 11 Feb 2009 05:04:46 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1754274AbZBKKEi (ORCPT ); Wed, 11 Feb 2009 05:04:38 -0500 Received: from mx3.mail.elte.hu ([157.181.1.138]:34524 "EHLO mx3.mail.elte.hu" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750746AbZBKKEh (ORCPT ); Wed, 11 Feb 2009 05:04:37 -0500 Date: Wed, 11 Feb 2009 11:04:32 +0100 From: Ingo Molnar To: Mandeep Singh Baines Cc: Frederic Weisbecker , linux-kernel@vger.kernel.org, rientjes@google.com, mbligh@google.com, thockin@google.com Subject: Re: [PATCH] softlockup: ensure the task has been switched out once Message-ID: <20090211100432.GF20518@elte.hu> References: <4991a557.1ade660a.582e.7835@mx.google.com> <20090210181504.GI7332@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20090210181504.GI7332@google.com> User-Agent: Mutt/1.5.18 (2008-05-17) X-ELTE-VirusStatus: clean X-ELTE-SpamScore: -1.5 X-ELTE-SpamLevel: X-ELTE-SpamCheck: no X-ELTE-SpamVersion: ELTE 2.0 X-ELTE-SpamCheck-Details: score=-1.5 required=5.9 tests=BAYES_00 autolearn=no SpamAssassin version=3.2.3 -1.5 BAYES_00 BODY: Bayesian spam probability is 0 to 1% [score: 0.0000] Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org * Mandeep Singh Baines wrote: > Frederic Weisbecker (fweisbec@gmail.com) wrote: > > When we check if a task has been switched out since the last scan, we might > > have a race condition on the following scenario: > > > > _ the task is freshly created and scheduled > > _ it puts its state to TASK_UNINTERRUPTIBLE and is not yet switched out > > _ check_hung_task() scans this task and will report a false positive because > > t->nvcsw + t->nivcsw == t->last_switch_count == 0 > > > > Add a check for such cases. > > > > Signed-off-by: Frederic Weisbecker > > --- > > kernel/hung_task.c | 8 +++++++- > > 1 files changed, 7 insertions(+), 1 deletions(-) > > > > diff --git a/kernel/hung_task.c b/kernel/hung_task.c > > index 0c924de..022a492 100644 > > --- a/kernel/hung_task.c > > +++ b/kernel/hung_task.c > > @@ -72,7 +72,13 @@ static void check_hung_task(struct task_struct *t, unsigned long timeout) > > { > > unsigned long switch_count = t->nvcsw + t->nivcsw; > > > > - if (t->flags & PF_FROZEN) > > + /* > > + * Ensure the task is not frozen. > > + * Also, when a freshly created task is scheduled once, changes > > + * its state to TASK_UNINTERRUPTIBLE without having ever been > > + * switched out once, it musn't be checked. > > + */ > > + if (unlikely(t->flags & PF_FROZEN || !switch_count)) > > return; > > > > if (switch_count != t->last_switch_count) { > > -- > > 1.6.1 > > > > > > > Looks good to me. Thanks for fixing this! > > Acked-by: Mandeep Singh Baines Applied to tip:core/softlockup, thanks guys! Ingo