linux-kernel - Re: lockdep splat in CPU hotplug

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Message-ID: <alpine.LNX.2.00.1410222255540.17725@pobox.suse.cz>
Date:	Wed, 22 Oct 2014 22:57:25 +0200 (CEST)
From:	Jiri Kosina <jkosina@...e.cz>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
cc:	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Pavel Machek <pavel@....cz>,
	Steven Rostedt <rostedt@...dmis.org>,
	Dave Jones <davej@...hat.com>,
	Daniel Lezcano <daniel.lezcano@...aro.org>,
	Nicolas Pitre <nico@...aro.org>, linux-kernel@...r.kernel.org,
	linux-pm@...r.kernel.org
Subject: Re: lockdep splat in CPU hotplug

On Wed, 22 Oct 2014, Paul E. McKenney wrote:

> rcu: More on deadlock between CPU hotplug and expedited grace periods
> 
> Commit dd56af42bd82 (rcu: Eliminate deadlock between CPU hotplug and
> expedited grace periods) was incomplete.  Although it did eliminate
> deadlocks involving synchronize_sched_expedited()'s acquisition of
> cpu_hotplug.lock via get_online_cpus(), it did nothing about the similar
> deadlock involving acquisition of this same lock via put_online_cpus().
> This deadlock became apparent with testing involving hibernation.
> 
> This commit therefore changes put_online_cpus() acquisition of this lock
> to be conditional, and increments a new cpu_hotplug.puts_pending field
> in case of acquisition failure.  Then cpu_hotplug_begin() checks for this
> new field being non-zero, and applies any changes to cpu_hotplug.refcount.
> 

Yes, this works. FWIW, please feel free to add 

	Reported-and-tested-by: Jiri Kosina <jkosina@...e.cz>

once merging it.

Why lockdep produced such an incomplete stacktrace still remains 
unexplained.

Thanks,

-- 
Jiri Kosina
SUSE Labs
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