Re: [RFC PATCH v3] mm: use READ_ONCE in page_cpupid_xchg_last()

[Vlastimil Babka]

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@xxxxxxxxxx>
>> Suggested-by: Christian Borntraeger <borntraeger@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 = 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?

> Or this doesn't matter?

I think it would matter.

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