Re: Possible deadlock related to CPU hotplug and kernfs

[Jiang Liu]

On 2015/9/4 4:08, Rafael J. Wysocki wrote:
> Hi Tejun,
> 
> On Thu, Sep 3, 2015 at 6:19 PM, Tejun Heo <tj@xxxxxxxxxx> 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?
Hi Rafael,
	I think your are right. The lock_device_hotplug_sysfs() has
already provided a solution for such a deadlock scenario. And there's
another related code path at boot as:
smp_init()
	->cpu_up()
		->cpu_hotplug_begin()
	So it seems to be a false alarm. Any way to teach lockdep
about this to get rid of the false alarm?
Thanks!
Gerry

> 
> Thanks,
> Rafael
> 
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