Re: Lockdep message on workqueue_mutex

From: Arjan van de Ven
Date: Tue Aug 22 2006 - 08:34:00 EST

On Tue, 2006-08-22 at 14:10 +0200, Heiko Carstens wrote:
> git commit 9b41ea7289a589993d3daabc61f999b4147872c4 causes the lockdep
> message below on cpu hotplug (git kernel of today).
> We have:
> cpu_down (takes cpu_add_remove_lock)
> blocking_notifier_call_chain (takes (cpu_chain).rwsem)
> workqueue_cpu_callback (takes workqueue_mutex)
> blocking_notifier_call_chain (releases (cpu_chain).rwsem)
> blocking_notifier_call_chain (takes (cpu_chain).rwsem)
> ^^^^^^^^^^^^^^^^^^^^^^^
> -> reverse locking order, since we still hold workqueue_mutex.
> But since all of this is protected by the cpu_add_remove_lock this looks
> legal. Well, at least it's safe as long as no other cpu callback function
> does anything that will take the workqueue_mutex as well.

so you're saying this locking is entirely redundant ? ;-)

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