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@xxxxxxx> 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 _______________________________________________ linux-pm mailing list linux-pm@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linux-foundation.org/mailman/listinfo/linux-pm
