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

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Message-ID: <alpine.DEB.2.20.1702101154480.29784@east.gentwo.org>
Date:   Fri, 10 Feb 2017 11:58:52 -0600 (CST)
From:   Christoph Lameter <cl@...ux.com>
To:     Michal Hocko <mhocko@...nel.org>
cc:     Thomas Gleixner <tglx@...utronix.de>,
        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>,
        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 Thu, 9 Feb 2017, Michal Hocko wrote:

> Christoph, you are completely ignoring the reality and the code. There
> is no need for stop_machine nor it is helping anything. As the matter
> of fact there is a synchronization with the cpu hotplug needed if you
> want to make a per-cpu specific operations. get_online_cpus is the
> most straightforward and heavy weight way to do this synchronization
> but not the only one. As the patch [1] describes we do not really need
> get_online_cpus in drain_all_pages because we can do _better_. But
> this is not in any way a generic thing applicable to other code paths.
>
> If you disagree then you are free to post patches but hand waving you
> are doing here is just wasting everybody's time. So please cut it here
> unless you have specific proposals to improve the current situation.

I am fine with the particular solution here for this particular problem.

My problem is the general way of having to synchronize via
get_online_cpus() because of cpu hotplug operations.