linux-kernel - Re: [PATCH v1] kthread/smpboot: Serialize kthread parking against wakeup

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Message-ID: <20180501104015.GE12235@hirez.programming.kicks-ass.net>
Date:   Tue, 1 May 2018 12:40:15 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Kohli, Gaurav" <gkohli@...eaurora.org>
Cc:     tglx@...utronix.de, mpe@...erman.id.au, mingo@...nel.org,
        bigeasy@...utronix.de, linux-kernel@...r.kernel.org,
        linux-arm-msm@...r.kernel.org,
        Neeraj Upadhyay <neeraju@...eaurora.org>,
        Will Deacon <will.deacon@....com>,
        Oleg Nesterov <oleg@...hat.com>
Subject: Re: [PATCH v1] kthread/smpboot: Serialize kthread parking against
 wakeup

On Tue, May 01, 2018 at 12:18:45PM +0200, Peter Zijlstra wrote:
> Aaaah... I think I've spotted a problem there. We clear SHOULD_PARK
> before we rebind, so if the thread lost the first PARKED store,
> does the completion, gets migrated, cycles through the loop and now
> observes !SHOULD_PARK and bails the wait-loop, then __kthread_bind()
> will forever wait.
> 
> Is that what you had in mind?

Another possible problem is concurrent thread_park(), if both observe
!IS_PARKED, we'll end up with 2 threads waiting on the completion, but
we only do a single complete().

Of course, this might not be a suppored use of the API, and I don't
think this will ever actually happen. But the whole !test_bit(IS_PARKED)
thing seems to suggest it is fine calling this on an already parked
thread.

Confusing stuff that should be cleared up in any case.