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

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Date:	Mon, 6 Apr 2009 20:50:38 +0530
From:	Gautham R Shenoy <ego@...ibm.com>
To:	Alan Stern <stern@...land.harvard.edu>
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, Apr 06, 2009 at 10:37:10AM -0400, Alan Stern wrote:
> 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.

Thanks for clarifying this Alan. I guess it boils down to teaching
lockdep about this false-positive.

> 
> Alan Stern

-- 
Thanks and Regards
gautham
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