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.
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