linux-kernel - Re: [PATCH] memcg: remove unneeded preempt

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Message-ID: <alpine.DEB.2.00.1108251413130.27407@router.home>
Date:	Thu, 25 Aug 2011 14:19:26 -0500 (CDT)
From:	Christoph Lameter <cl@...ux.com>
To:	Peter Zijlstra <peterz@...radead.org>
cc:	James Bottomley <James.bottomley@...senPartnership.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Greg Thelen <gthelen@...gle.com>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Balbir Singh <bsingharora@...il.com>,
	Daisuke Nishimura <nishimura@....nes.nec.co.jp>,
	linux-arch@...r.kernel.org
Subject: Re: [PATCH] memcg: remove unneeded preempt_disable

On Thu, 25 Aug 2011, Peter Zijlstra wrote:

> Also, I thought this_cpu thing's were at best locally atomic. If you
> make them full blown atomic ops then even __this_cpu ops will have to be
> full atomic ops, otherwise:
>
>
> CPU0			CPU(1)
>
> this_cpu_inc(&foo);	preempt_disable();
> 			__this_cpu_inc(&foo);
> 			preempt_enable();
>
> might step on each other's toes.

They would both have their own instance of "foo". per cpu atomicity is
only one requirement of this_cpu_ops. The other is the ability to relocate
accesses relative to the current per cpu area.

Full blown atomicity is almost a superset of per cpu atomicity but its
only usable if the full atomic instructions can also relocate accesses
relative to some base.

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