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Date: Fri, 10 Feb 2017 11:58:52 -0600 (CST)
From: Christoph Lameter <cl@linux.com>
Subject: Re: mm: deadlock between get_online_cpus/pcpu_alloc
In-Reply-To: <20170209191547.GA31906@dhcp22.suse.cz>
Message-ID: <alpine.DEB.2.20.1702101154480.29784@east.gentwo.org>
References: <20170208152106.GP5686@dhcp22.suse.cz> <alpine.DEB.2.20.1702081011460.4938@east.gentwo.org> <alpine.DEB.2.20.1702081838560.3536@nanos> <alpine.DEB.2.20.1702082109530.13608@east.gentwo.org> <alpine.DEB.2.20.1702091240000.3604@nanos>
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Sender: owner-linux-mm@kvack.org
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To: Michal Hocko <mhocko@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>, 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, Michal Hocko wrote:

> Christoph, you are completely ignoring the reality and the code. There
> is no need for stop_machine nor it is helping anything. As the matter
> of fact there is a synchronization with the cpu hotplug needed if you
> want to make a per-cpu specific operations. get_online_cpus is the
> most straightforward and heavy weight way to do this synchronization
> but not the only one. As the patch [1] describes we do not really need
> get_online_cpus in drain_all_pages because we can do _better_. But
> this is not in any way a generic thing applicable to other code paths.
>
> If you disagree then you are free to post patches but hand waving you
> are doing here is just wasting everybody's time. So please cut it here
> unless you have specific proposals to improve the current situation.

I am fine with the particular solution here for this particular problem.

My problem is the general way of having to synchronize via
get_online_cpus() because of cpu hotplug operations.

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