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 <[email protected]> >> --- >> 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.
