Re: Possible deadlock related to CPU hotplug and kernfs

[Rafael J. Wysocki]

Hi,

On Fri, Sep 4, 2015 at 9:20 AM, Jiang Liu <jiang.liu@xxxxxxxxxxxxxxx> wrote:
> 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?

Well, maybe we could call lock_device_hotplug() from that code path too?

Thanks,
Rafael
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