linux-kernel - Re: [PATCH] Revert "mm/page_alloc: protect pcp->batch accesses with ACCESS

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Date:	Wed, 30 Mar 2016 13:10:44 +0200
From:	Michal Hocko <mhocko@...nel.org>
To:	Hekuang <hekuang@...wei.com>
Cc:	Mel Gorman <mgorman@...hsingularity.net>,
	akpm@...ux-foundation.org, vbabka@...e.cz, rientjes@...gle.com,
	cody@...ux.vnet.ibm.com, gilad@...yossef.com,
	kosaki.motohiro@...il.com, mgorman@...e.de, penberg@...nel.org,
	lizefan@...wei.com, wangnan0@...wei.com, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Revert "mm/page_alloc: protect pcp->batch accesses with
 ACCESS_ONCE"

On Wed 30-03-16 18:51:12, Hekuang wrote:
> hi
> 
> 在 2016/3/30 18:38, Mel Gorman 写道:
> >On Wed, Mar 30, 2016 at 10:22:07AM +0000, He Kuang wrote:
> >>This reverts commit 998d39cb236fe464af86a3492a24d2f67ee1efc2.
> >>
> >>When local irq is disabled, a percpu variable does not change, so we can
> >>remove the access macros and let the compiler optimize the code safely.
> >>
> >batch can be changed from other contexts. Why is this safe?
> >
> I've mistakenly thought that per_cpu variable can only be accessed by that
> cpu.

git blame would point you to 998d39cb236f ("mm/page_alloc: protect
pcp->batch accesses with ACCESS_ONCE"). I haven't looked into the code
deeply to confirm this is still the case but it would be a good lead
that this is not that simple. ACCESS_ONCE resp. {READ,WRITE}_ONCE are
usually quite subtle so I would encourage you or anybody else who try to
remove them to study the code and the history deeper before removing
them.

-- 
Michal Hocko
SUSE Labs