From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753854Ab1HSVHU (ORCPT ); Fri, 19 Aug 2011 17:07:20 -0400 Received: from ogre.sisk.pl ([217.79.144.158]:53642 "EHLO ogre.sisk.pl" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752686Ab1HSVHS (ORCPT ); Fri, 19 Aug 2011 17:07:18 -0400 From: "Rafael J. Wysocki" To: Tejun Heo Subject: Re: [PATCH 01/16] freezer: fix current->state restoration race in refrigerator() Date: Fri, 19 Aug 2011 23:08:59 +0200 User-Agent: KMail/1.13.6 (Linux/3.1.0-rc2+; KDE/4.6.0; x86_64; ; ) Cc: menage@google.com, linux-kernel@vger.kernel.org, arnd@arndb.de, oleg@redhat.com References: <1313763382-12341-1-git-send-email-tj@kernel.org> <1313763382-12341-2-git-send-email-tj@kernel.org> In-Reply-To: <1313763382-12341-2-git-send-email-tj@kernel.org> MIME-Version: 1.0 Content-Type: Text/Plain; charset="iso-8859-2" Content-Transfer-Encoding: 7bit Message-Id: <201108192308.59705.rjw@sisk.pl> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Friday, August 19, 2011, Tejun Heo wrote: > refrigerator() saves current->state before entering frozen state and > restores it before returning using __set_current_state(); however, > this is racy, for example, please consider the following sequence. > > set_current_state(TASK_INTERRUPTIBLE); > try_to_sleep(); Did you mean try_to_freeze() here? > if (kthread_should_stop()) > break; > schedule(); > > If kthread_stop() races with ->state restoration, the restoration can > restore ->state to TASK_INTERRUPTIBLE after kthread_stop() sets it to > TASK_RUNNING but kthread_should_stop() may still see zero > ->should_stop because there's no memory barrier between restoring > TASK_INTERRUPTIBLE and kthread_should_stop() test. > > This isn't restricted to kthread_should_stop(). current->state is > often used in memory barrier based synchronization and silently > restoring it w/o mb breaks them. > > Use set_current_state() instead. > > Signed-off-by: Tejun Heo > --- > kernel/freezer.c | 8 +++++++- > 1 files changed, 7 insertions(+), 1 deletions(-) > > diff --git a/kernel/freezer.c b/kernel/freezer.c > index 7b01de9..575f863 100644 > --- a/kernel/freezer.c > +++ b/kernel/freezer.c > @@ -58,7 +58,13 @@ void refrigerator(void) > current->flags &= ~PF_FREEZING; > > pr_debug("%s left refrigerator\n", current->comm); > - __set_current_state(save); > + > + /* > + * Restore saved task state before returning. The mb'd version > + * needs to be used; otherwise, it might silently break > + * synchronization which depends on ordered task state change. > + */ > + set_current_state(save); I think the change is correct. > } > EXPORT_SYMBOL(refrigerator); Thanks, Rafael