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

[Christian Borntraeger]

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@xxxxxxxxxx>
>> ---
>>  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.