Re: Lockdep message on workqueue_mutex

[Andrew Morton <akpm@osdl.org>]

On Tue, 22 Aug 2006 14:36:32 +0200
Arjan van de Ven <arjan@infradead.org> 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 ? ;-)

Nope, not all code paths which access the data which is protected by
workqueue_mutex take cpu_add_remove_lock.

Simplifying it, we have:

	blocking_notifier_call_chain()
	->down_read(cpu_chain.rwsem)
	->workqueue_cpu_callback()
	  ->mutex_lock(workqueue_mutex)
	->up_read(cpu_chain.rwsem)

	blocking_notifier_call_chain()
	->down_read(cpu_chain.rwsem)
	->workqueue_cpu_callback()
	  ->mutex_unlock(workqueue_mutex)
	->up_read(cpu_chain.rwsem)

Which is OK as long as nobody runs cpu_up() or cpu_down() while holding
workqueue_mutex.