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Message-ID: <alpine.DEB.2.20.1702091240000.3604@nanos>
Date: Thu, 9 Feb 2017 12:42:44 +0100 (CET)
From: Thomas Gleixner <tglx@...utronix.de>
To: Christoph Lameter <cl@...ux.com>
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 Wed, 8 Feb 2017, Christoph Lameter wrote:
> On Wed, 8 Feb 2017, Thomas Gleixner wrote:
>
> > There is a world outside yours. Hotplug is actually used frequently for
> > power purposes in some scenarios.
>
> The usual case does not inolve hotplug.
We do not care about your definition of "usual". The kernel serves _ALL_
use cases.
> > It will improve nothing. The stop machine context is extremly limited and
> > you cannot do complex things there at all. Not to talk about the inability
> > of taking a simple mutex which would immediately deadlock the machine.
>
> You do not need to do complex things. Basically flipping some cpu mask
> bits will do it. stop machine ensures that code is not
> executing on the processors when the bits are flipped. That will ensure
> that there is no need to do any get_online_cpu() nastiness in critical VM
> paths since we are guaranteed not to be executing them.
And how does that solve the problem at hand? Not at all:
CPU 0 CPU 1
for_each_online_cpu(cpu)
==> cpu = 1
stop_machine()
set_cpu_online(1, false)
queue_work(cpu1)
Thanks,
tglx
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