[PATCH 2/3] mm/page_ref: Ensure page_ref_unfreeze is ordered against prior accesses

[Will Deacon]

page_ref_freeze and page_ref_unfreeze are designed to be used as a pair,
wrapping a critical section where struct pages can be modified without
having to worry about consistency for a concurrent fast-GUP.

Whilst page_ref_freeze has full barrier semantics due to its use of
atomic_cmpxchg, page_ref_unfreeze is implemented using atomic_set, which
doesn't provide any barrier semantics and allows the operation to be
reordered with respect to page modifications in the critical section.

This patch ensures that page_ref_unfreeze is ordered after any critical
section updates, by invoking smp_mb__before_atomic() prior to the
atomic_set.

Cc: "Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx>
Acked-by: Steve Capper <steve.capper@xxxxxxx>
Signed-off-by: Will Deacon <will.deacon@xxxxxxx>
---
 include/linux/page_ref.h | 1 +
 1 file changed, 1 insertion(+)

diff --git a/include/linux/page_ref.h b/include/linux/page_ref.h
index 610e13271918..74d32d7905cb 100644
--- a/include/linux/page_ref.h
+++ b/include/linux/page_ref.h
@@ -174,6 +174,7 @@ static inline void page_ref_unfreeze(struct page *page, int count)
 	VM_BUG_ON_PAGE(page_count(page) != 0, page);
 	VM_BUG_ON(count == 0);
 
+	smp_mb__before_atomic();
 	atomic_set(&page->_refcount, count);
 	if (page_ref_tracepoint_active(__tracepoint_page_ref_unfreeze))
 		__page_ref_unfreeze(page, count);
-- 
2.1.4