Re: Possible deadlock related to CPU hotplug and kernfs

[Tejun Heo <tj@kernel.org>]

On Tue, Sep 01, 2015 at 03:12:34PM +0800, Jiang Liu wrote:
> Hi Rafael and Tejun,
> 	When running CPU hotplug tests, it triggers an lockdep warning
> as follow. The two possible deadlock paths are:
> 1) echo x > /sys/devices/system/cpu/cpux/online
>    ->kernfs_fop_write()
>      ->kernfs_get_active()
> 1.a)   ->rwsem_acquire_read(&kn->dep_map, 0, 1, _RET_IP_);
>          ->cpu_up()
> 1.b)       ->cpu_hotplug_begin()[lock_map_acquire(&cpu_hotplug.dep_map)]
> 2) hardware triggers hotplug evetns
>    ->acpi_device_hotplug()
>      ->acpi_processor_remove()
> 2.a)   ->cpu_hotplug_begin()[lock_map_acquire(&cpu_hotplug.dep_map)]
>          ->unregister_cpu()
>            ->device_del()
>              ->kernfs_remove_by_name_ns()
>                ->__kernfs_remove()
>                  ->kernfs_drain()
> 2.b)               ->rwsem_acquire(&kn->dep_map, 0, 0, _RET_IP_)
> 
> So there is a possible deadlock scenario among 1.a, 1.b, 2.a and 2.b.
> I'm not familiar with kernfs, so could you please help to comment:
> 1) whether is a real deadlock issue?

Yes, it seems to be.  It's highly unlikely but still possible.

> 2) any recommended way to get it fixed?

This usually happens with "delete" files and it's worked around by
performing special self-removal on the file before actually removing
the device.  I suppose on/offline files would need to turn off
active_protection with kernfs_[un]break_active_protection() which
should probably grow sysfs and device layer wrappers.

Thanks.

-- 
tejun