From: Michal Hocko <mhocko@kernel.org>
To: Hekuang <hekuang@huawei.com>
Cc: Mel Gorman <mgorman@techsingularity.net>,
akpm@linux-foundation.org, vbabka@suse.cz, rientjes@google.com,
cody@linux.vnet.ibm.com, gilad@benyossef.com,
kosaki.motohiro@gmail.com, mgorman@suse.de, penberg@kernel.org,
lizefan@huawei.com, wangnan0@huawei.com, linux-mm@kvack.org,
linux-kernel@vger.kernel.org
Subject: Re: [PATCH] Revert "mm/page_alloc: protect pcp->batch accesses with ACCESS_ONCE"
Date: Wed, 30 Mar 2016 13:10:44 +0200 [thread overview]
Message-ID: <20160330111044.GA4324@dhcp22.suse.cz> (raw)
In-Reply-To: <56FBAFA0.3010604@huawei.com>
On Wed 30-03-16 18:51:12, Hekuang wrote:
> hi
>
> a?? 2016/3/30 18:38, Mel Gorman a??e??:
> >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
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next prev parent reply other threads:[~2016-03-30 11:10 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-03-30 10:22 [PATCH] Revert "mm/page_alloc: protect pcp->batch accesses with ACCESS_ONCE" He Kuang
2016-03-30 10:38 ` Mel Gorman
2016-03-30 10:51 ` Hekuang
2016-03-30 11:10 ` Michal Hocko [this message]
2016-03-31 1:14 ` Hekuang
2016-03-31 1:39 ` Zefan Li
2016-03-31 1:47 ` Hekuang
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