Re: [PATCH] mm, oom: Fix race when selecting process to kill

From: Sameer Nanda
Date: Thu Nov 07 2013 - 14:35:16 EST

On Wed, Nov 6, 2013 at 4:35 PM, David Rientjes <rientjes@xxxxxxxxxx> wrote:
> On Wed, 6 Nov 2013, Sameer Nanda wrote:
>> David -- I think we can make the duration that the tasklist_lock is
>> held smaller by consolidating the process selection logic that is
>> currently split across select_bad_process and oom_kill_process into
>> one place in select_bad_process. The tasklist_lock would then need to
>> be held only when the thread lists are being traversed. Would you be
>> ok with that? I can re-spin the patch if that sounds like a workable
>> option.
> No, this caused hundreds of machines to hit soft lockups for Google
> because there's no synchronization that prevents dozens of cpus to take
> tasklist_lock in the oom killer during parallel memcg oom conditions and
> never allow the write_lock_irq() on fork() or exit() to make progress. We
> absolutely must hold tasklist_lock for as little time as possible in the
> oom killer.
> That said, I've never actually seen your reported bug manifest in our
> production environment so let's see if Oleg has any ideas.

Is the path you are referring to mem_cgroup_out_of_memory calling
oom_kill_process? If so, then that path doesn't appear to suffer from
the two step select_bad_process, oom_kill_process race since
mem_cgroup_out_of_memory directly calls oom_kill_process without going
through select_bad_process. This also means that the patch I sent is
incorrect since it removes the existing tasklist_lock protection in

Respinning patch to take care of this case.

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