Re: mm: deadlock between get_online_cpus/pcpu_alloc

[Mel Gorman <mgorman@techsingularity.net>]

On Tue, Feb 07, 2017 at 04:34:59PM +0100, Michal Hocko wrote:
> > But we do not care about the whole cpu hotplug code. The only part we
> > really do care about is the race inside drain_pages_zone and that will
> > run in an atomic context on the specific CPU.
> > 
> > You are absolutely right that using the mutex is safe as well but the
> > hotplug path is already littered with locks and adding one more to the
> > picture doesn't sound great to me. So I would really like to not use a
> > lock if that is possible and safe (with a big fat comment of course).
> 
> And with the full changelog. I hope I haven't missed anything this time.
> ---
> From 8c6af3116520251cc4ec2213f0a4ed2544bb4365 Mon Sep 17 00:00:00 2001
> From: Michal Hocko <mhocko@suse.com>
> Date: Tue, 7 Feb 2017 16:08:35 +0100
> Subject: [PATCH] mm, page_alloc: do not depend on cpu hotplug locks inside the
>  allocator
> 
> <SNIP>
>
> Reported-by: Dmitry Vyukov <dvyukov@google.com>
> Signed-off-by: Michal Hocko <mhocko@suse.com>

Not that I can think of. It's almost identical to the diff I posted with
the exception of the mutex in the cpu hotplug teardown path. I agree that
in the current implementation that it should be unnecessary even if I
thought it would be more robust against any other hotplug churn.

Acked-by: Mel Gorman <mgorman@techsingularity.net>

-- 
Mel Gorman
SUSE Labs

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