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Date: Thu, 9 Feb 2017 12:42:44 +0100 (CET)
From: Thomas Gleixner <tglx@linutronix.de>
To: Christoph Lameter <cl@linux.com>
cc: Michal Hocko <mhocko@kernel.org>,
        Mel Gorman <mgorman@techsingularity.net>,
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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, 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