linux-kernel - Re: Spurious lockdep splat in v4.15-rc9

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Message-ID: <20180122174031.GG2228@hirez.programming.kicks-ass.net>
Date:   Mon, 22 Jan 2018 18:40:31 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Tejun Heo <tj@...nel.org>
Cc:     Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: Spurious lockdep splat in v4.15-rc9

On Mon, Jan 22, 2018 at 09:10:18AM -0800, Tejun Heo wrote:
> Hello, Peter, Ingo.
> 
> I get the below lockdep warning if I try to write a config into cgroup
> cpu.max file.  It's warning about A-A deadlock, but it's obviously
> spurious - the system doesn't lock up and the warning is about two
> get_online_cpus() calls nesting.

Looks real, just not instantly fatal. It would generate an actual
deadlock the moment there is a contending hotplug operation around
though.

I'll have a wee look.