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Date: Tue, 7 Feb 2017 23:25:17 +0100 (CET)
From: Thomas Gleixner <tglx@linutronix.de>
Subject: Re: mm: deadlock between get_online_cpus/pcpu_alloc
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References: <2cdef192-1939-d692-1224-8ff7d7ff7203@suse.cz> <20170207102809.awh22urqmfrav5r6@techsingularity.net> <20170207103552.GH5065@dhcp22.suse.cz> <20170207113435.6xthczxt2cx23r4t@techsingularity.net> <20170207114327.GI5065@dhcp22.suse.cz>
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To: Christoph Lameter <cl@linux.com>
Cc: Michal Hocko <mhocko@kernel.org>, Mel Gorman <mgorman@techsingularity.net>, Vlastimil Babka <vbabka@suse.cz>, Dmitry Vyukov <dvyukov@google.com>, Tejun Heo <tj@kernel.org>, "linux-mm@kvack.org" <linux-mm@kvack.org>, LKML <linux-kernel@vger.kernel.org>, Ingo Molnar <mingo@kernel.org>, Peter Zijlstra <peterz@infradead.org>, syzkaller <syzkaller@googlegroups.com>, Andrew Morton <akpm@linux-foundation.org>

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.

Thanks,

	tglx

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