On Wed 07-12-16 10:40:47, Christian Borntraeger wrote: > On 12/07/2016 10:29 AM, Vlastimil Babka wrote: > > 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@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? > >> > >> 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. > > That change would make it clearer, but the code is correct anyway, > as assignments in C are done from right to left, so > old_flags = flags = READ_ONCE(page->flags); > > is equivalent to > > flags = READ_ONCE(page->flags); > old_flags = flags; OK, I guess you are right. For some reason I thought that the compiler is free to bypass flags and split an assignment a = b = c; into b = c; a = c which would still follow from right to left rule. I guess I am over speculating here though, so sorry for the noise. -- Michal Hocko SUSE Labs -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx"> email@xxxxxxxxx </a>