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Date:   Thu, 21 Sep 2023 09:45:38 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     John Stultz <jstultz@google.com>
Cc:     Waiman Long <longman@redhat.com>, Ingo Molnar <mingo@redhat.com>,
        Will Deacon <will.deacon@arm.com>,
        Boqun Feng <boqun.feng@gmail.com>, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2] locking/semaphore: Use wake_q to wake up processes
 outside lock critical section
Message-ID: <20230921074538.GD14803@noisy.programming.kicks-ass.net>
References: <20220909192848.963982-1-longman@redhat.com>
 <CANDhNCpsDbz9ZT+h4Zcq-7QMJ-uqs3rW+MXCVcbi2A2hnTjBpQ@mail.gmail.com>
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On Wed, Sep 20, 2023 at 05:05:56PM -0700, John Stultz wrote:
> On Fri, Sep 9, 2022 at 12:28 PM Waiman Long <longman@redhat.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!

https://git.kernel.org/pub/scm/linux/kernel/git/peterz/queue.git/log/?h=debug/experimental

and use with: earlyprintk=serial,ttyS0,115200 force_early_printk, or
somesuch.

I could not have done perf or much of the sched patches without it.