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 S1751155AbdE3Wh0 (ORCPT <rfc822;w@1wt.eu>);
        Tue, 30 May 2017 18:37:26 -0400
Received: from mail-pf0-f170.google.com ([209.85.192.170]:33420 "EHLO
        mail-pf0-f170.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
        with ESMTP id S1750992AbdE3WhZ (ORCPT
        <rfc822;linux-kernel@vger.kernel.org>);
        Tue, 30 May 2017 18:37:25 -0400
From: Junaid Shahid <junaids@google.com>
To: linux-kernel@vger.kernel.org, akpm@linux-foundation.org
Cc: andreslc@google.com, gthelen@google.com, rusty@rustcorp.com.au
Subject: Re: [PATCH] kthread: Fix race condition between kthread_parkme() and kthread_unpark()
Date: Tue, 30 May 2017 15:37:23 -0700
Message-ID: <4713763.QsSBOgQyOh@js-desktop.svl.corp.google.com>
Organization: Google
User-Agent: KMail/4.13.3 (Linux/4.4.0-75-generic; KDE/4.13.3; x86_64; ; )
In-Reply-To: <20170429023236.60452-1-junaids@google.com>
References: <20170429023236.60452-1-junaids@google.com>
MIME-Version: 1.0
Content-Transfer-Encoding: 7Bit
Content-Type: text/plain; charset="us-ascii"
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

(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);
>