[PATCH] mm/mmzone.c: fix page_cpupid_xchg_last() to READ_ONCE() the page flags

[Peter Collingbourne]

After submitting a patch with a compare-exchange loop similar to this
one to set the KASAN tag in the page flags, Andrey Konovalov pointed
out that we should be using READ_ONCE() to read the page flags. Fix
it here.

Fixes: 75980e97dacc ("mm: fold page->_last_nid into page->flags where possible")
Signed-off-by: Peter Collingbourne <pcc@xxxxxxxxxx>
Link: https://linux-review.googlesource.com/id/I2e1f5b5b080ac9c4e0eb7f98768dba6fd7821693
Cc: stable@xxxxxxxxxxxxxxx
---
 mm/mmzone.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/mmzone.c b/mm/mmzone.c
index eb89d6e018e2..f84b84b0d3fc 100644
--- a/mm/mmzone.c
+++ b/mm/mmzone.c
@@ -90,7 +90,7 @@ 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);
-- 
2.34.1.703.g22d0c6ccf7-goog