Re: INFO: possible circular locking dependency at cleanup_workqueue_thread

[Johannes Berg <johannes@sipsolutions.net>]

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On Tue, 2009-05-19 at 18:09 +0200, Oleg Nesterov wrote:

> > Right. But exactly this happens in the hibernate case --
> 
> not sure I understand your "exactly this" ;)
> 
> But your explanation of the deadlock below looks great!

Yeah... I got side-tracked, I had a scenario in mind that actually
needed cpu_add_remove_lock().

> except I don't understand how cpu_add_remove_lock makes the difference...
> And thus I can't understand why cpu_down() (called lockless) have the
> same problems. Please see below.
> 
> > Anyway, you can have a deadlock like this:
> >
> > CPU 3			CPU 2				CPU 1
> > 							suspend/hibernate
> > 			something:
> > 			rtnl_lock()			device_pm_lock()
> > 							-> mutex_lock(&dpm_list_mtx)
> >
> > 			mutex_lock(&dpm_list_mtx)
> >
> > linkwatch_work
> >  -> rtnl_lock()
> > 							disable_nonboot_cpus()
> 
> let's suppose disable_nonboot_cpus() does not take cpu_add_remove_lock,
> 
> > 							-> flush CPU 3 workqueue
> 
> in this case the deadlock is still here?
> 
> We can't flush because we hold the lock (dpm_list_mtx) which depends
> on another lock taken by work->func(), the "classical" bug with flush.
> 
> No?

Yeah, it looks like cpu_add_remove_lock doesn't make a difference...
It's just lockdep reporting a longer chain that also leads to a
deadlock. OTOH just replace dpm_list_mtx with cpu_add_remove_lock and
you have the same scenario... happens too, I guess, somehow.

johannes

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