[lkml]   [2015]   [Sep]   [3]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
Messages in this thread
SubjectRe: Possible deadlock related to CPU hotplug and kernfs
Hi Tejun,

On Thu, Sep 3, 2015 at 6:19 PM, Tejun Heo <> wrote:
> Hello, Rafael.
> On Thu, Sep 03, 2015 at 02:58:16AM +0200, Rafael J. Wysocki wrote:
>> So acpi_device_hotplug() calls lock_device_hotplug() which simply
>> acquires device_hotplug_lock. It is held throughout the entire
>> hot-add/hot-remove code path.
>> Witing anything to /sys/devices/system/cpu/cpux/online goes through
>> online_store() in drivers/base/core.c and that does
>> lock_device_hotplug_sysfs() which then attempts to acquire
>> device_hotplug_lock using mutex_trylock(). And it only calls
>> either device_online() or device_offline() if it ends up with the
>> lock held.
>> Quite frankly, I don't see how these particular two code paths can
>> deadlock in any way.
>> So either a third code path is involved which is not executed
>> under device_hotplug_lock, or lockdep needs to be told to actually
>> take device_hotplug_lock into account in this case IMO.
> Hmm... all sysfs rw functions are protected from removal. ie. by
> default, removal of a sysfs file drains in-flight rw operations, so
> the hot plug path grabs a lock and then tries to remove a file and
> writing to the online file makes the file's write method to try to
> grab the same lock. It deadlocks if the hotunplug path already has
> the lock and trying to drain the online file for removal.

My point is that you cannot get into that situation. If hotplug
already holds device_hotplug_lock, the write to "online" will end up
doing restart_syscall().

If the "online" code path is holding the lock, hotplug cannot acquire
it and cannot proceed.

Am I missing anything?


 \ /
  Last update: 2015-09-03 22:21    [W:0.081 / U:52.460 seconds]
©2003-2018 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site