linux-kernel - Re: [linux-pm] pm-hibernate : possible circular locking dependency detected

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Date:	Mon, 6 Apr 2009 14:42:26 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Gautham R Shenoy <ego@...ibm.com>
cc:	"Rafael J. Wysocki" <rjw@...k.pl>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Linux Kernel List <linux-kernel@...r.kernel.org>,
	Linux-pm mailing list <linux-pm@...ts.linux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [linux-pm] pm-hibernate : possible circular locking dependency
 detected

On Mon, 6 Apr 2009, Gautham R Shenoy wrote:

> > If I understand correctly it isn't really a deadlock scenario, but it
> > is a lockdep violation.  The violation is:
> > 
> > 	The pci_device_probe() path 2) proves that dpm_list_mtx [4] can
> > 	be acquired while cpu_hotplug.lock [3] is held;
> > 
> > 	The hibernate() path 3) proves that cpu_hotplug.lock [3] can be
> > 	acquired while dpm_list_mtx [4] is held.
> > 
> > The two pathways cannot run simultaneously (and hence cannot deadlock) 
> > because the prepare() stage of hibernation is supposed to stop all 
> > device probing.  But lockdep will still report a problem.
> 
> Thanks for clarifying this Alan. I guess it boils down to teaching
> lockdep about this false-positive.

Or else changing the code somehow to avoid the violation completely.  
But I have no idea how...  And AFAIK, teaching lockdep about special 
cases like this is not so easy to do.

Alan Stern

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