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Date: Thu, 9 Feb 2017 08:00:57 -0600 (CST)
From: Christoph Lameter <cl@linux.com>
Subject: Re: mm: deadlock between get_online_cpus/pcpu_alloc
In-Reply-To: <alpine.DEB.2.20.1702091240000.3604@nanos>
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References: <20170207123708.GO5065@dhcp22.suse.cz> <20170207135846.usfrn7e4znjhmogn@techsingularity.net> <20170207141911.GR5065@dhcp22.suse.cz> <20170207153459.GV5065@dhcp22.suse.cz> <20170207162224.elnrlgibjegswsgn@techsingularity.net> <20170207164130.GY5065@dhcp22.suse.cz>
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Sender: owner-linux-mm@kvack.org
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To: Thomas Gleixner <tglx@linutronix.de>
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 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.


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