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

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Message-Id: <1239093831.798.5717.camel@twins>
Date:	Tue, 07 Apr 2009 10:43:50 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	"Rafael J. Wysocki" <rjw@...k.pl>
Cc:	Alan Stern <stern@...land.harvard.edu>,
	Gautham R Shenoy <ego@...ibm.com>,
	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, 2009-04-06 at 21:58 +0200, Rafael J. Wysocki wrote:
> On Monday 06 April 2009, Alan Stern wrote:
> > 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.
> 
> Yeah, I've just wanted to ask about that.  Peter, how can we do it?

I think it would come down to modeling that blocking of probes as a lock
or something -- because that's basically what it is.

So on the regular probe path, take a read lock of this lock, and on the
suspend path take it as write or something.

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