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Date: Wed, 7 Dec 2016 09:43:05 +0100
From: Michal Hocko <mhocko@kernel.org>
Subject: Re: [RFC PATCH v3] mm: use READ_ONCE in page_cpupid_xchg_last()
Message-ID: <20161207084305.GA20350@dhcp22.suse.cz>
References: <584523E4.9030600@huawei.com>
 <58461A0A.3070504@huawei.com>
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To: Xishi Qiu <qiuxishi@huawei.com>
Cc: Andrew Morton <akpm@linux-foundation.org>, Mel Gorman <mgorman@techsingularity.net>, Yaowei Bai <baiyaowei@cmss.chinamobile.com>, Christian Borntraeger <borntraeger@de.ibm.com>, Linux MM <linux-mm@kvack.org>, LKML <linux-kernel@vger.kernel.org>, Yisheng Xie <xieyisheng1@huawei.com>

On Tue 06-12-16 09:53:14, Xishi Qiu wrote:
> A compiler could re-read "old_flags" from the memory location after reading
> and calculation "flags" and passes a newer value into the cmpxchg making 
> the comparison succeed while it should actually fail.
> 
> Signed-off-by: Xishi Qiu <qiuxishi@huawei.com>
> Suggested-by: Christian Borntraeger <borntraeger@de.ibm.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 = READ_ONCE(page->flags);
>  		last_cpupid = page_cpupid_last(page);

what prevents compiler from doing?
		old_flags = READ_ONCE(page->flags);
		flags = READ_ONCE(page->flags);

Or this doesn't matter?

>  
>  		flags &= ~(LAST_CPUPID_MASK << LAST_CPUPID_PGSHIFT);
> -- 
> 1.8.3.1
> 

-- 
Michal Hocko
SUSE Labs

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