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    SubjectRe: [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@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.

    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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