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

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Date:	Mon, 6 Apr 2009 10:37:10 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	"Rafael J. Wysocki" <rjw@...k.pl>
cc:	ego@...ibm.com, 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, Rafael J. Wysocki wrote:

> On Monday 06 April 2009, Gautham R Shenoy wrote:
> > On Sun, Apr 05, 2009 at 03:44:54PM +0200, Ingo Molnar wrote:
> > > 
> > > * Rafael J. Wysocki <rjw@...k.pl> wrote:
> > > 
> > > > On Sunday 05 April 2009, Ming Lei wrote:
> > > > > kernel version : one simple usb-serial patch against  commit
> > > > > 6bb597507f9839b13498781e481f5458aea33620.
> > > > > 
> > > > > Thanks.
> > > > 
> > > > Hmm, CPU hotplug again, it seems.
> > > > 
> > > > I'm not sure who's the maintainer at the moment.  Andrew, is that 
> > > > Gautham?
> > > 
> > > CPU hotplug tends to land on the scheduler people's desk normally.
> > > 
> > > But i'm not sure that's the real thing here - key appears to be this 
> > > work_on_cpu() worklet by the cpufreq code:
> > 
> > Actually, there are two dependency chains here which can lead to a deadlock.
> > The one we're seeing here is the longer of the two.
> > 
> > If the relevant locks are numbered as follows:
> > [1]: cpu_policy_rwsem
> > [2]: work_on_cpu
> > [3]: cpu_hotplug.lock
> > [4]: dpm_list_mtx
> > 
> > 
> > The individual callpaths are:
> > 
> > 1) do_dbs_timer()[1] --> dbs_check_cpu() --> __cpufreq_driver_getavg()
> >                                                                   |
> >                       work_on_cpu()[2] <-- get_measured_perf() <--|
> > 
> > 
> > 2) pci_device_probe() --> .. --> pci_call_probe() [3] --> work_on_cpu()[2]
> >                                                                      |
> >                   [4] device_pm_add() <-- ..<-- local_pci_probe() <--|
> 
> This should block on [4] held by hibernate().  That's why it calls
> device_pm_lock() after all.
> 
> > 3) hibernate() --> hibernatioin_snapshot() --> create_image()
> >                                                           |
> >        disable_nonboot_cpus() <-- [4] device_pm_lock() <--|
> >        |
> >        |--> _cpu_down() [3] --> cpufreq_cpu_callback() [1]
> > 
> > 
> > The two chains which can deadlock are
> > 
> > a) [1] --> [2] --> [4] --> [3] --> [1] (The one in this log)
> > and
> > b) [3] --> [2] --> [4] --> [3]
> 
> What exactly is the b) scenario?

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.

Alan Stern

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