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Date: Wed, 8 Feb 2017 09:11:06 -0600 (CST)
From: Christoph Lameter <cl@linux.com>
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To: Michal Hocko <mhocko@kernel.org>
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        Peter Zijlstra <peterz@infradead.org>,
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        Andrew Morton <akpm@linux-foundation.org>
Subject: Re: mm: deadlock between get_online_cpus/pcpu_alloc
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On Wed, 8 Feb 2017, Michal Hocko wrote:

> > Huch? stop_machine() is horrible and heavy weight. Don't go there, there
> > must be simpler solutions than that.
>
> Absolutely agreed. We are in the page allocator path so using the
> stop_machine* is just ridiculous. And, in fact, there is a much simpler
> solution [1]

That is nonsense. stop_machine would be used when adding removing a
processor. There would be no need to synchronize when looping over active
cpus anymore. get_online_cpus() etc would be removed from the hot
path since the cpu masks are guaranteed to be stable.