Re: lockdep splat in CPU hotplug

[Jiri Kosina]

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@xxxxxxx>

once merging it.

Why lockdep produced such an incomplete stacktrace still remains 
unexplained.

Thanks,

-- 
Jiri Kosina
SUSE Labs
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