linux-kernel - Re: mm: deadlock between get_online_cpus/pcpu

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Date:   Tue, 7 Feb 2017 16:22:24 +0000
From:   Mel Gorman <mgorman@...hsingularity.net>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     Vlastimil Babka <vbabka@...e.cz>,
        Dmitry Vyukov <dvyukov@...gle.com>, Tejun Heo <tj@...nel.org>,
        Christoph Lameter <cl@...ux.com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        syzkaller <syzkaller@...glegroups.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: mm: deadlock between get_online_cpus/pcpu_alloc

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@...e.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@...gle.com>
> Signed-off-by: Michal Hocko <mhocko@...e.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@...hsingularity.net>

-- 
Mel Gorman
SUSE Labs