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Date: Wed, 8 Feb 2017 08:35:28 +0100
From: Michal Hocko <mhocko@kernel.org>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: Christoph Lameter <cl@linux.com>,
        Mel Gorman <mgorman@techsingularity.net>,
        Vlastimil Babka <vbabka@suse.cz>, Dmitry Vyukov <dvyukov@google.com>,
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        Peter Zijlstra <peterz@infradead.org>,
        syzkaller <syzkaller@googlegroups.com>,
        Andrew Morton <akpm@linux-foundation.org>
Subject: Re: mm: deadlock between get_online_cpus/pcpu_alloc
Message-ID: <20170208073527.GA5686@dhcp22.suse.cz>
References: <20170207113435.6xthczxt2cx23r4t@techsingularity.net>
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On Tue 07-02-17 23:25:17, Thomas Gleixner wrote:
> On Tue, 7 Feb 2017, Christoph Lameter wrote:
> > 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.
> 
> 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]

[1] http://lkml.kernel.org/r/20170207201950.20482-1-mhocko@kernel.org
-- 
Michal Hocko
SUSE Labs