Date: Mon, 23 Nov 2009 08:31:40 -0600 (CST)
From: Christoph Lameter <cl@linux-foundation.org>
To: "Zhang, Yanmin" <yanmin_zhang@linux.intel.com>
cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>,
       "hugh.dickins@tiscali.co.uk" <hugh.dickins@tiscali.co.uk>,
       linux-mm@kvack.org, linux-kernel@vger.kernel.org,
       akpm@linux-foundation.org, Tejun Heo <tj@kernel.org>,
       Andi Kleen <andi@firstfloor.org>
Subject: Re: [MM] Make mm counters per cpu instead of atomic
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On Mon, 23 Nov 2009, Zhang, Yanmin wrote:

> Another theoretic issue is below scenario:
> Process A get the read lock on cpu 0 and is scheduled to cpu 2 to unlock. Then
> it's scheduled back to cpu 0 to repeat the step. eventually, the reader counter
> will overflow. Considering multiple thread cases, it might be faster to
> overflow than what we imagine. When it overflows, processes will hang there.

True.... We need to find some alternative to per cpu data to scale mmap
sem then.
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