From: Junaid Shahid <junaids@google.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>, linux-kernel@vger.kernel.org,
        Andres Lagar-Cavilla <andreslc@google.com>,
        Greg Thelen <gthelen@google.com>,
        Rusty Russell <rusty@rustcorp.com.au>
Subject: Re: [PATCH] kthread: Fix race condition between kthread_parkme() and kthread_unpark()
Date: Fri, 29 Sep 2017 13:58:28 -0700
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Thanks for the clarification. But in that case, shouldn’t the patch check whether IS_PARKED was already set before calling complete(&self->parked)? Otherwise, the completion count for self->parked could be more than 1 as a result of spurious wakeups, which could make a future call to kthread_park complete prematurely.

Thanks,
Junaid

On Friday, September 29, 2017 10:28:38 AM Peter Zijlstra wrote:
> 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.