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

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Date:   Tue, 7 Feb 2017 10:55:51 -0600 (CST)
From:   Christoph Lameter <cl@...ux.com>
To:     Michal Hocko <mhocko@...nel.org>
cc:     Mel Gorman <mgorman@...hsingularity.net>,
        Vlastimil Babka <vbabka@...e.cz>,
        Dmitry Vyukov <dvyukov@...gle.com>, Tejun Heo <tj@...nel.org>,
        "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, 7 Feb 2017, Michal Hocko wrote:

> I am always nervous when seeing hotplug locks being used in low level
> code. It has bitten us several times already and those deadlocks are
> quite hard to spot when reviewing the code and very rare to hit so they
> tend to live for a long time.

Yep. Hotplug events are pretty significant. Using stop_machine_XXXX() etc
would be advisable and that would avoid the taking of locks and get rid of all the
ocmplexity, reduce the code size and make the overall system much more
reliable.

Thomas?