Re: INFO: possible circular locking dependency at cleanup_workqueue_thread

[Rafael J. Wysocki]

On Friday 22 May 2009, Johannes Berg wrote:
> On Tue, 2009-05-19 at 20:51 +0200, Oleg Nesterov wrote:
> 
> > > > > 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.
> > 
> > So. we should not call cpu_down/disable_nonboot_cpus under device_pm_lock().
> > 
> > At first glance this was changed by
> > 
> > 	PM: Change hibernation code ordering
> > 	4aecd6718939eb3c4145b248369b65f7483a8a02
> > 
> > 	PM: Change suspend code ordering
> > 	900af0d973856d6feb6fc088c2d0d3fde57707d3
> > 
> > commits. Rafael, could you take a look?
> 
> I just arrived at the same conclusion, heh. I can't say I understand
> these changes though, the part about calling the platform differently
> may make sense, but calling why disable non-boot CPUs at a different
> place?

Because the ordering of platform callbacks and cpu[_up()|_down()] is also
important, at least on resume.

In principle we can call device_pm_unlock() right before calling
disable_nonboot_cpus() and take the lock again right after calling
enable_nonboot_cpus(), if that helps.

Thanks,
Rafael
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