linux-kernel - Re: lockdep splat in CPU hotplug

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Date:	Wed, 22 Oct 2014 14:09:43 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Jiri Kosina <jkosina@...e.cz>
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, Oct 22, 2014 at 10:57:25PM +0200, Jiri Kosina wrote:
> 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.

Done, and thank you for both the bug report and the testing!

> Why lockdep produced such an incomplete stacktrace still remains 
> unexplained.

On that, I must defer to people more familiar with stack frames.

							Thanx, Paul

> Thanks,
> 
> -- 
> Jiri Kosina
> SUSE Labs
> 

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