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

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Message-ID: <96939.1313677618@turing-police.cc.vt.edu>
Date:	Thu, 18 Aug 2011 10:26:58 -0400
From:	Valdis.Kletnieks@...edu
To:	Johannes Weiner <jweiner@...hat.com>
Cc:	Greg Thelen <gthelen@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	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>
Subject: Re: [PATCH] memcg: remove unneeded preempt_disable

On Thu, 18 Aug 2011 11:38:00 +0200, Johannes Weiner said:

> Note that on non-x86, these operations themselves actually disable and
> reenable preemption each time, so you trade a pair of add and sub on
> x86
> 
> -	preempt_disable()
> 	__this_cpu_xxx()
> 	__this_cpu_yyy()
> -	preempt_enable()
> 
> with
> 
> 	preempt_disable()
> 	__this_cpu_xxx()
> +	preempt_enable()
> +	preempt_disable()
> 	__this_cpu_yyy()
> 	preempt_enable()
> 
> everywhere else.

That would be an unexpected race condition on non-x86, if you expected _xxx and
_yyy to be done together without a preempt between them. Would take mere
mortals forever to figure that one out. :)


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