[RFC v2 10/10] mm: use atomic_cmpxchg_acquire in page_ref_freeze().

[Pasha Tatashin]

page_ref_freeze and page_ref_unfreeze are designed to be used as a pair.
They protect critical sections where struct page can be modified.

page_ref_unfreeze() is protected by _release() atomic operation, but
page_ref_freeze() is not as it is assumed that cmpxch provides the full
barrier.

Instead, use the appropriate atomic_cmpxchg_acquire() to ensure that
memory model is excplicitly followed.

Signed-off-by: Pasha Tatashin <pasha.tatashin@xxxxxxxxxx>
---
 include/linux/page_ref.h | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/include/linux/page_ref.h b/include/linux/page_ref.h
index 26676d3bcd58..ecd92d7f3eef 100644
--- a/include/linux/page_ref.h
+++ b/include/linux/page_ref.h
@@ -308,7 +308,8 @@ static inline bool folio_try_get_rcu(struct folio *folio)
 
 static inline int page_ref_freeze(struct page *page, int count)
 {
-	int ret = likely(atomic_cmpxchg(&page->_refcount, count, 0) == count);
+	int old_val = atomic_cmpxchg_acquire(&page->_refcount, count, 0);
+	int ret = likely(old_val == count);
 
 	if (page_ref_tracepoint_active(page_ref_freeze))
 		__page_ref_freeze(page, count, ret);
-- 
2.34.0.rc1.387.gb447b232ab-goog