linux-kernel - Re: [RFC PATCH] mm: use ACCESS_ONCE in page_cpupid_xchg

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Message-Id: <26c66f28-d836-4d6e-fb40-3e2189a540ed@de.ibm.com>
Date:   Mon, 5 Dec 2016 09:31:18 +0100
From:   Christian Borntraeger <borntraeger@...ibm.com>
To:     Xishi Qiu <qiuxishi@...wei.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Yaowei Bai <baiyaowei@...s.chinamobile.com>
Cc:     Linux MM <linux-mm@...ck.org>, LKML <linux-kernel@...r.kernel.org>,
        Yisheng Xie <xieyisheng1@...wei.com>
Subject: Re: [RFC PATCH] mm: use ACCESS_ONCE in page_cpupid_xchg_last()

On 12/05/2016 09:23 AM, Xishi Qiu wrote:
> By reading the code, I find the following code maybe optimized by
> compiler, maybe page->flags and old_flags use the same register,
> so use ACCESS_ONCE in page_cpupid_xchg_last() to fix the problem.

please use READ_ONCE instead of ACCESS_ONCE for future patches.

> 
> Signed-off-by: Xishi Qiu <qiuxishi@...wei.com>
> ---
>  mm/mmzone.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/mmzone.c b/mm/mmzone.c
> index 5652be8..e0b698e 100644
> --- a/mm/mmzone.c
> +++ b/mm/mmzone.c
> @@ -102,7 +102,7 @@ int page_cpupid_xchg_last(struct page *page, int cpupid)
>  	int last_cpupid;
> 
>  	do {
> -		old_flags = flags = page->flags;
> +		old_flags = flags = ACCESS_ONCE(page->flags);
>  		last_cpupid = page_cpupid_last(page);
> 
>  		flags &= ~(LAST_CPUPID_MASK << LAST_CPUPID_PGSHIFT);


I dont thing that this is actually a problem. The code below does  

   } while (unlikely(cmpxchg(&page->flags, old_flags, flags) != old_flags))

and the cmpxchg should be an atomic op that should already take care of everything
(page->flags is passed as a pointer).