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

[Xishi Qiu]

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