linux-kernel - Re: [PATCH] THP: fix comment about memory barrier

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Date:	Fri, 5 Apr 2013 12:52:24 -0700 (PDT)
From:	David Rientjes <rientjes@...gle.com>
To:	Minchan Kim <minchan@...nel.org>
cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Mel Gorman <mgorman@...e.de>, Hugh Dickins <hughd@...gle.com>,
	Kamezawa Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [PATCH] THP: fix comment about memory barrier

On Fri, 5 Apr 2013, Minchan Kim wrote:

> Now, memory barrier in __do_huge_pmd_anonymous_page doesn't work.
> Because lru_cache_add_lru uses pagevec so it could miss spinlock
> easily so above rule was broken so user might see inconsistent data.
> 
> I was not first person who pointed out the problem. Mel and Peter
> pointed out a few months ago and Peter pointed out further that
> even spin_lock/unlock can't make sure it.
> http://marc.info/?t=134333512700004
> 
> 	In particular:
> 
>         	*A = a;
>         	LOCK
>         	UNLOCK
>         	*B = b;
> 
> 	may occur as:
> 
>         	LOCK, STORE *B, STORE *A, UNLOCK
> 
> At last, Hugh pointed out that even we don't need memory barrier
> in there because __SetPageUpdate already have done it from
> Nick's [1] explicitly.
> 
> So this patch fixes comment on THP and adds same comment for
> do_anonymous_page, too because everybody except Hugh was missing
> that. It means we needs COMMENT about that.
> 
> [1] 0ed361dec "mm: fix PageUptodate data race"
> 
> Cc: Mel Gorman <mgorman@...e.de>
> Cc: Hugh Dickins <hughd@...gle.com>
> Cc: Kamezawa Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
> Cc: David Rientjes <rientjes@...gle.com>
> Cc: Peter Zijlstra <a.p.zijlstra@...llo.nl>
> Acked-by: Andrea Arcangeli <aarcange@...hat.com>
> Signed-off-by: Minchan Kim <minchan@...nel.org>

Acked-by: David Rientjes <rientjes@...gle.com>
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