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

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Message-ID: <20170704150106.GA11168@linux-80c1.suse>
Date:   Tue, 4 Jul 2017 08:01:06 -0700
From:   Davidlohr Bueso <dave@...olabs.net>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>, linux-mm@...ck.org,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Michal Hocko <mhocko@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [patch V2 2/2] mm/memory-hotplug: Switch locking to a percpu
 rwsem

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