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SubjectRe: [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@sisk.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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