linux-kernel - Re: lockdep splat in v5.11-rc1 involving console

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Message-ID: <X/Xa1fwplnZIOm+U@hirez.programming.kicks-ass.net>
Date:   Wed, 6 Jan 2021 16:44:21 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Paul E. McKenney" <paulmck@...nel.org>
Cc:     tglx@...utronix.de, linux-kernel@...r.kernel.org,
        valentin.schneider@....com, bristot@...hat.com, frederic@...nel.org
Subject: Re: lockdep splat in v5.11-rc1 involving console_sem and rq locks

On Wed, Jan 06, 2021 at 06:46:21AM -0800, Paul E. McKenney wrote:
> Huh.  The WARN does not always generate the lockdep complaint.  But
> fair enough.

Any printk()/WARN/BUG with rq lock held ought to generate that splat,
sometimes we die before we splat. The printk rewrite should eventually
fix that.

> >   https://lkml.kernel.org/r/20201226025117.2770-1-jiangshanlai@gmail.com
> 
> Thomas pointed me at this one a couple of weeks ago.  Here is an
> additional fix for rcutorture: f67e04bb0695 ("torture: Break affinity
> of kthreads last running on outgoing CPU").  I am still getting WARNs
> and lockdep splats with both applied.

That patch looks racy, what avoids the task being shuffled right back
before the CPU goes offline?

> What would break if I made the code dump out a few entries in the
> runqueue if the warning triggered?

There was a patch around that did that, Valentin might remember where
that was.