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

From: Michal Hocko
Date: Wed Mar 30 2016 - 07:11:01 EST

On Wed 30-03-16 18:51:12, Hekuang wrote:
> hi
> å 2016/3/30 18:38, Mel Gorman åé:
> >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

Michal Hocko