linux-kernel - Re: [PATCH RT v2] Fix a lockup in wait_for

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Message-ID: <20190510120824.GL16145@minyard.net>
Date:   Fri, 10 May 2019 07:08:24 -0500
From:   Corey Minyard <minyard@....org>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     linux-rt-users@...r.kernel.org, linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>, tglx@...utronix.de,
        Steven Rostedt <rostedt@...dmis.org>,
        Corey Minyard <cminyard@...sta.com>
Subject: Re: [PATCH RT v2] Fix a lockup in wait_for_completion() and friends

On Fri, May 10, 2019 at 12:33:18PM +0200, Sebastian Andrzej Siewior wrote:
> On 2019-05-09 14:33:20 [-0500], minyard@....org wrote:
> > From: Corey Minyard <cminyard@...sta.com>
> > 
> > The function call do_wait_for_common() has a race condition that
> > can result in lockups waiting for completions.  Adding the thread
> > to (and removing the thread from) the wait queue for the completion
> > is done outside the do loop in that function.  However, if the thread
> > is woken up, the swake_up_locked() function will delete the entry
> > from the wait queue.  If that happens and another thread sneaks
> > in and decrements the done count in the completion to zero, the
> > loop will go around again, but the thread will no longer be in the
> > wait queue, so there is no way to wake it up.
> 
> applied, thank you.
> 
> Sebastian

Thanks a bunch.  Do I need to do anything for backports?  I'm pretty
sure this goes back to 4.4.

-corey