linux-kernel - Re: [PATCH] kthread: Fix race condition between kthread_parkme() and kthread

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Message-ID: <20170929082838.z2eyp67hbdfcpibj@hirez.programming.kicks-ass.net>
Date:   Fri, 29 Sep 2017 10:28:38 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Junaid Shahid <junaids@...gle.com>, linux-kernel@...r.kernel.org,
        Andres Lagar-Cavilla <andreslc@...gle.com>,
        Greg Thelen <gthelen@...gle.com>,
        Rusty Russell <rusty@...tcorp.com.au>
Subject: Re: [PATCH] kthread: Fix race condition between kthread_parkme() and
 kthread_unpark()

On Fri, Sep 29, 2017 at 09:59:55AM +0200, Thomas Gleixner wrote:
> On Thu, 28 Sep 2017, Junaid Shahid wrote:
> 
> > Hi Peter,
> > 
> > It looks like try_cmpxchg is not available on non-x86 archs, but other than
> > that the version that you proposed looks good.
> > 
> > One thing that I am a bit curious about is that the original code, before
> > either patch, had a test_and_set_bit for KTHREAD_IS_PARKED rather than just
> > a set_bit. I can't think of any reason why that was needed, since it
> > doesn't look like TASK_PARKED tasks are susceptible to spurious wakeups. Do
> > you by any chance happen to know if there was any specific reason for it?
> 
> Everything is susceptible to spurious wakeups and has to deal with it.

Right, we should code as if they are at all times possible. Currently,
for TASK_PARKED, I don't think they can happen, but I've had patches
that introduce them on purpose (regardless the state) just to stress the
code.

IIRC only TASK_STOPPED and/or TASK_TRACED hard rely on not getting any.