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Date: Thu, 9 Feb 2017 09:42:09 -0600 (CST)
From: Christoph Lameter <cl@linux.com>
Subject: Re: mm: deadlock between get_online_cpus/pcpu_alloc
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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:

> > The stop_machine would need to ensure that all cpus cease processing
> > before proceeding.
>
> Ok. I try again:
>
> CPU 0	     	  	    CPU 1
> for_each_online_cpu(cpu)
>   ==> cpu = 1
>  			    stop_machine()
>
> Stops processing on all CPUs by preempting the current execution and
> forcing them into a high priority busy loop with interrupts disabled.

Exactly that means we are outside of the sections marked with
get_online_cpous().

> It does exactly what you describe. It stops processing on all other cpus
> until release, but that does not invalidate any data on those cpus.

Why would it need to invalidate any data? The change of the cpu masks
would need to be done when the machine is stopped. This sounds exactly
like what we need and much of it is already there.

Lets get rid of get_online_cpus() etc.

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