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Date: Thu, 9 Feb 2017 08:00:57 -0600 (CST)
From: Christoph Lameter <cl@linux.com>
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To: Thomas Gleixner <tglx@linutronix.de>
cc: Michal Hocko <mhocko@kernel.org>,
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Subject: Re: mm: deadlock between get_online_cpus/pcpu_alloc
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On Thu, 9 Feb 2017, Thomas Gleixner wrote:

> 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,

Well thats not how I remember stop_machine does work. Doesnt it stop the
processing on all cpus otherwise its not a real usable stop.

The stop_machine would need to ensure that all cpus cease processing
before proceeding.