Re: INFO: possible circular locking dependency at cleanup_workqueue_thread

[Rafael J. Wysocki]

On Sunday 24 May 2009, Alan Stern wrote:
> On Sun, 24 May 2009, Rafael J. Wysocki wrote:
> 
> > The patch is appended for reference (Alan, please have a look; I can't recall
> > why exactly we have called device_pm_lock() from the core suspend/hibernation
> > code instead of acquiring the lock locally in drivers/base/power/main.c) and
> > I'll attach it to the bug entry too.
> 
> I can't remember the reason either.  Probably there wasn't any.  The 
> patch looks fine, and it has the nice added benefit that now the only 
> user of device_pm_lock() will be device_move().
>
> > ---
> > From: Rafael J. Wysocki <rjw@xxxxxxx>
> > Subject: PM: Do not hold dpm_list_mtx while disabling/enabling nonboot CPUs
> > 
> > We shouldn't hold dpm_list_mtx while executing
> > [disable|enable]_nonboot_cpus(), because theoretically this may lead
> > to a deadlock as shown by the following example (provided by Johannes
> > Berg):
> > 
> > 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()
> >                                        -> flush CPU 3 workqueue
> > 
> > Fortunately, device drivers are supposed to stop any activities that
> > might lead to the registration of new device objects and/or to the
> > removal of the existing ones way before disable_nonboot_cpus() is
> 
> Strictly speaking, drivers are still allowed to unregister existing 
> devices.  They are forbidden only to register new ones.  This shouldn't 
> hurt anything, though.

You're right, I'll fix the changelog.
 
> > called, so it shouldn't be necessary to hold dpm_list_mtx over the
> > entire late part of device suspend and early part of device resume.
> > 
> > Thus, during the late suspend and the early resume of devices acquire
> > dpm_list_mtx only when dpm_list is going to be traversed and release
> > it right after that.
> 
> Acked-by: Alan Stern <stern@xxxxxxxxxxxxxxxxxxx>

Thanks!

Best,
Rafael
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