linux-kernel - Re: Lockdep message on workqueue

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Message-ID: <20060822135327.GB29577@osiris.boeblingen.de.ibm.com>
Date:	Tue, 22 Aug 2006 15:53:27 +0200
From:	Heiko Carstens <heiko.carstens@...ibm.com>
To:	Arjan van de Ven <arjan@...radead.org>
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, Aug 22, 2006 at 02:36:32PM +0200, Arjan van de Ven wrote:
> 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 ? ;-)

No, I'm just saying that I think that it currently cannot deadlock. But I
think the workqueue cpu hotplug code should be changed, so that it doesn't
return with the workqueue_mutex being held.
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