linux-kernel - Re: [PATCH v2] locking/semaphore: Use wake_q to wake up processes outside lock critical section

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Message-ID: <CANDhNCpsDbz9ZT+h4Zcq-7QMJ-uqs3rW+MXCVcbi2A2hnTjBpQ@mail.gmail.com>
Date:   Wed, 20 Sep 2023 17:05:56 -0700
From:   John Stultz <jstultz@...gle.com>
To:     Waiman Long <longman@...hat.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Will Deacon <will.deacon@....com>,
        Boqun Feng <boqun.feng@...il.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] locking/semaphore: Use wake_q to wake up processes
 outside lock critical section

On Fri, Sep 9, 2022 at 12:28 PM Waiman Long <longman@...hat.com> wrote:
>
> It was found that a circular lock dependency can happen with the
> following locking sequence:
>
>    +--> (console_sem).lock --> &p->pi_lock --> &rq->__lock --+
>    |                                                         |
>    +---------------------------------------------------------+
>
> The &p->pi_lock --> &rq->__lock sequence is very common in all the
> task_rq_lock() calls.

Thanks for sending this out! I've been hitting these lockdep warningns
a lot recently, particularly if I have any debug printks/WARN_ONs in
the scheduler that trip, so I'm eager to get a fix for this!

That said, running with your patch, I'm seeing bootup hang pretty
close to where init starts when I've had a fair amount of debug
printks go off. It's odd because the lockup detectors don't seem to
fire.
I'll try to debug further, but wanted to give you a heads up. Let me
know if you have any suggestions.

thanks
-john