linux-kernel - Re: Lockdep message on workqueue

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Message-Id: <1156250192.2976.47.camel@laptopd505.fenrus.org>
Date:	Tue, 22 Aug 2006 14:36:32 +0200
From:	Arjan van de Ven <arjan@...radead.org>
To:	Heiko Carstens <heiko.carstens@...ibm.com>
Cc:	Andrew Morton <akpm@...l.org>, Greg KH <gregkh@...e.de>,
	Dave Jones <davej@...emonkey.org.uk>,
	linux-kernel@...r.kernel.org
Subject: Re: Lockdep message on workqueue_mutex

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)
> [CPU_DOWN_PREPARE]
> blocking_notifier_call_chain (takes (cpu_chain).rwsem)
> workqueue_cpu_callback (takes workqueue_mutex)
> blocking_notifier_call_chain (releases (cpu_chain).rwsem)
> [CPU_DEAD]
> 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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