linux-kernel - Re: PI futexes + lock stealing woes

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Message-ID: <87y3mf8f1j.fsf@ni.com>
Date:   Wed, 06 Dec 2017 20:09:28 -0600
From:   Gratian Crisan <gratian.crisan@...com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Julia Cartwright <julia@...com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Gratian Crisan <gratian.crisan@...com>,
        linux-kernel@...r.kernel.org, Darren Hart <dvhart@...radead.org>,
        Ingo Molnar <mingo@...hat.com>
Subject: Re: PI futexes + lock stealing woes

Peter Zijlstra writes:

> On Wed, Nov 29, 2017 at 11:56:05AM -0600, Julia Cartwright wrote:
>
>> fixup_owner() used to have additional seemingly relevant checks in place
>> that were removed 73d786bd043eb ("futex: Rework inconsistent
>> rt_mutex/futex_q state").
>
> *groan*... yes. I completely missed that extra case also applied to
> requeue_pi (requeue always did hurt my brain).

FWIW I have been testing for about two days now with the fixup_owner()
hunk of 73d786bd043eb ("futex: Rework inconsistent rt_mutex/futex_q
state") reverted. So far it hasn't hit the race/deadlock. It normally
takes around 8 hours to reproduce.

I've also tried Julia's msleep() trick for expanding the race window for
the last 4 hours or so of testing and it seems to be still going.

Thanks,
    Gratian