Re: Lockdep message on workqueue_mutex

[Andrew Morton <akpm@osdl.org>]

On Tue, 22 Aug 2006 15:53:27 +0200
Heiko Carstens <heiko.carstens@de.ibm.com> wrote:

> 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.

That's deliberate: it's to prevent the workqueue code from walking
cpu_online_map while it is in the process of being changed.