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

[John Stultz]

On Fri, Sep 9, 2022 at 12:28 PM Waiman Long <longman@xxxxxxxxxx> 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