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

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Message-ID: <alpine.DEB.2.20.1702080906100.3955@east.gentwo.org>
Date:   Wed, 8 Feb 2017 09:06:47 -0600 (CST)
From:   Christoph Lameter <cl@...ux.com>
To:     Thomas Gleixner <tglx@...utronix.de>
cc:     Michal Hocko <mhocko@...nel.org>,
        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 Tue, 7 Feb 2017, Thomas Gleixner wrote:

> > 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.
>
> Huch? stop_machine() is horrible and heavy weight. Don't go there, there
> must be simpler solutions than that.

Inserting or removing hardware is a heavy process. This would help quite a
bit with these issues for loops over active cpus.