Re: [patch V2 2/2] mm/memory-hotplug: Switch locking to a percpu rwsem

[Davidlohr Bueso <dave@stgolabs.net>]

On Tue, 04 Jul 2017, Thomas Gleixner wrote:

>Andrey reported a potential deadlock with the memory hotplug lock and the
>cpu hotplug lock.
>
>The reason is that memory hotplug takes the memory hotplug lock and then
>calls stop_machine() which calls get_online_cpus(). That's the reverse lock
>order to get_online_cpus(); get_online_mems(); in mm/slub_common.c
>
>The problem has been there forever. The reason why this was never reported
>is that the cpu hotplug locking had this homebrewn recursive reader writer
>semaphore construct which due to the recursion evaded the full lock dep
>coverage. The memory hotplug code copied that construct verbatim and
>therefor has similar issues.
>
>Three steps to fix this:
>
>1) Convert the memory hotplug locking to a per cpu rwsem so the potential
>   issues get reported proper by lockdep.

I particularly like how the mem hotplug is well suited for pcpu-rwsem.
As a side effect you end up optimizing get/put_online_mems() at the cost
of more overhead for the actual hotplug operation, which is rare and of less
performance importance.

Thanks,
Davidlohr

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