linux-kernel - Re: WARNING: possible circular locking dependency detected

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Message-ID: <alpine.DEB.2.20.1708251813550.1883@nanos>
Date:   Fri, 25 Aug 2017 18:21:05 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Byungchul Park <max.byungchul.park@...il.com>
cc:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Borislav Petkov <bp@...en8.de>,
        Byungchul Park <byungchul.park@....com>,
        Peter Zijlstra <peterz@...radead.org>,
        lkml <linux-kernel@...r.kernel.org>, kernel-team@....com
Subject: Re: WARNING: possible circular locking dependency detected

On Sat, 26 Aug 2017, Byungchul Park wrote:
> On Fri, Aug 25, 2017 at 11:47 PM, Sebastian Andrzej Siewior
> <bigeasy@...utronix.de> wrote:
> > We hold the sparse_irq_lock lock while waiting for the completion in the
> > CPU-down case and in the CPU-up case we acquire the sparse_irq_lock lock
> > while the other CPU is waiting for the completion.
> > This is not an issue if my interpretation of lockdep here is correct.
> 
> Hello Sebastian,
> 
> I think you parsed the message correctly.
> 
> The message is saying that, for example:
> 
> context A (maybe being up?)
> --
> lock(sparse_irq_lock) // wait for sparse_irq_lock in B to be released
> complete(st->done) // impossible to hit here
> 
> context B (maybe wanting to synchronize with the cpu being up?)
> --
> lock(sparse_irq_lock) // acquired successfully
> wait_for_completion(st->done) // wait for completion of st->done in A
> unlock(sparse_irq_lock) // impossible to hit here
> 
> I cannot check the kernel code at the moment.. I wonder if this scenario is
> impossible. Could you answer it?

Yes, it's impossible because cpu hotplug is globally serialized. So the cpu
down scenario cannot happen in parallel with the cpu up scenario.

Thanks,

	tglx