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SubjectRe: Possible deadlock related to CPU hotplug and kernfs
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


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