linux-kernel - Re: [RFC PATCH v3] mm: use READ_ONCE in page_cpupid_xchg

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Message-ID: <b3c3cff5-5d47-7a32-9def-9f42640c9211@suse.cz>
Date:   Wed, 7 Dec 2016 10:29:44 +0100
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     Xishi Qiu <qiuxishi@...wei.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Yaowei Bai <baiyaowei@...s.chinamobile.com>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Linux MM <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Yisheng Xie <xieyisheng1@...wei.com>
Subject: Re: [RFC PATCH v3] mm: use READ_ONCE in page_cpupid_xchg_last()

On 12/07/2016 09:58 AM, Michal Hocko wrote:
> On Wed 07-12-16 09:48:52, Vlastimil Babka wrote:
>> On 12/07/2016 09:43 AM, Michal Hocko wrote:
>>> 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@...wei.com>
>>>> Suggested-by: Christian Borntraeger <borntraeger@...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);
>>
>> AFAIK, READ_ONCE tells the compiler that page->flags is volatile. It
>> can't read from volatile location more times than being told?
> 
> But those are two different variables which we assign to so what
> prevents the compiler from applying READ_ONCE on each of them
> separately?

I would naively expect that it's assigned to flags first, and then from
flags to old_flags. But I don't know exactly the C standard evaluation
rules that apply here.

> Anyway, this could be addressed easily by

Yes, that way there should be no doubt.

> diff --git a/mm/mmzone.c b/mm/mmzone.c
> index 5652be858e5e..b4e093dd24c1 100644
> --- a/mm/mmzone.c
> +++ b/mm/mmzone.c
> @@ -102,10 +102,10 @@ int page_cpupid_xchg_last(struct page *page, int cpupid)
>  	int last_cpupid;
>  
>  	do {
> -		old_flags = flags = page->flags;
> +		old_flags = READ_ONCE(page->flags);
>  		last_cpupid = page_cpupid_last(page);
>  
> -		flags &= ~(LAST_CPUPID_MASK << LAST_CPUPID_PGSHIFT);
> +		flags = old_flags & ~(LAST_CPUPID_MASK << LAST_CPUPID_PGSHIFT);
>  		flags |= (cpupid & LAST_CPUPID_MASK) << LAST_CPUPID_PGSHIFT;
>  	} while (unlikely(cmpxchg(&page->flags, old_flags, flags) != old_flags));
>  
> 
>>> Or this doesn't matter?
>>
>> I think it would matter.
>>
>>>>  
>>>>  		flags &= ~(LAST_CPUPID_MASK << LAST_CPUPID_PGSHIFT);
>>>> -- 
>>>> 1.8.3.1
>>>>
>>>
>