Re: [RFC PATCH] mm: use ACCESS_ONCE in page_cpupid_xchg_last()

[Xishi Qiu <qiuxishi@huawei.com>]

On 2016/12/5 16:50, Christian Borntraeger wrote:

> On 12/05/2016 09:31 AM, Christian Borntraeger wrote:
>> 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@huawei.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).
>>
> 
> Reading the code again, you might be right, but I think your patch description
> is somewhat misleading. I think the problem is that old_flags and flags are
> not necessarily the same.
> 
> So what about
> 
> 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.
> 

Hi Christian,

I'll resend v2, thanks!

> 
> 



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