From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
        id S1751316AbdE3WkE (ORCPT <rfc822;w@1wt.eu>);
        Tue, 30 May 2017 18:40:04 -0400
Received: from mail.linuxfoundation.org ([140.211.169.12]:42598 "EHLO
        mail.linuxfoundation.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
        with ESMTP id S1751002AbdE3WkD (ORCPT
        <rfc822;linux-kernel@vger.kernel.org>);
        Tue, 30 May 2017 18:40:03 -0400
Date: Tue, 30 May 2017 15:40:02 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: Junaid Shahid <junaids@google.com>
Cc: linux-kernel@vger.kernel.org, andreslc@google.com, gthelen@google.com,
        rusty@rustcorp.com.au, Thomas Gleixner <tglx@linutronix.de>,
        Tejun Heo <tj@kernel.org>
Subject: Re: [PATCH] kthread: Fix race condition between kthread_parkme()
 and kthread_unpark()
Message-Id: <20170530154002.010800f3d2f74777ab9801f2@linux-foundation.org>
In-Reply-To: <4713763.QsSBOgQyOh@js-desktop.svl.corp.google.com>
References: <20170429023236.60452-1-junaids@google.com>
        <4713763.QsSBOgQyOh@js-desktop.svl.corp.google.com>
X-Mailer: Sylpheed 3.4.1 (GTK+ 2.24.23; x86_64-pc-linux-gnu)
Mime-Version: 1.0
Content-Type: text/plain; charset=US-ASCII
Content-Transfer-Encoding: 7bit
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

(cc tglx & tj)

On Tue, 30 May 2017 15:37:23 -0700 Junaid Shahid <junaids@google.com> wrote:

> (Resending)
> 
> On Friday, April 28, 2017 07:32:36 PM Junaid Shahid wrote:
> > In general, if kthread_unpark() and kthread_parkme() execute together,
> > the kthread is supposed to be in an unparked state. This is because
> > kthread_unpark() either wakes up the thread if it already got parked,
> > or it cancels a prior kthread_park() call and hence renders the
> > kthread_parkme() moot.
> > 
> > However, if kthread_unpark() happens to execute between the time that
> > kthread_parkme() checks the KTHREAD_SHOULD_STOP flag and sets the
> > KTHREAD_IS_PARKED flag, then kthread_unpark() will not wake up the
> > thread and it will remain in a parked state.
> > 
> > This is fixed by making the checking of KTHREAD_SHOULD_STOP and the
> > setting of KTHREAD_IS_PARKED atomic via a cmpxchg inside kthread_parkme.
> > 
> > Signed-off-by: Junaid Shahid <junaids@google.com>
> > ---
> >  kernel/kthread.c | 16 ++++++++++++----
> >  1 file changed, 12 insertions(+), 4 deletions(-)
> > 
> > diff --git a/kernel/kthread.c b/kernel/kthread.c
> > index 26db528c1d88..651f03baf62f 100644
> > --- a/kernel/kthread.c
> > +++ b/kernel/kthread.c
> > @@ -169,12 +169,20 @@ void *kthread_probe_data(struct task_struct *task)
> >  
> >  static void __kthread_parkme(struct kthread *self)
> >  {
> > +	ulong flags;
> > +
> >  	__set_current_state(TASK_PARKED);
> > -	while (test_bit(KTHREAD_SHOULD_PARK, &self->flags)) {
> > -		if (!test_and_set_bit(KTHREAD_IS_PARKED, &self->flags))
> > -			complete(&self->parked);
> > -		schedule();
> > +	flags = self->flags;
> > +
> > +	while (test_bit(KTHREAD_SHOULD_PARK, &flags)) {
> > +		if (cmpxchg(&self->flags, flags,
> > +			    flags | (1 << KTHREAD_IS_PARKED)) == flags) {
> > +			if (!test_bit(KTHREAD_IS_PARKED, &flags))
> > +				complete(&self->parked);
> > +			schedule();
> > +		}
> >  		__set_current_state(TASK_PARKED);
> > +		flags = self->flags;
> >  	}
> >  	clear_bit(KTHREAD_IS_PARKED, &self->flags);
> >  	__set_current_state(TASK_RUNNING);
> >