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.shute...@linux.intel.com>
Acked-by: Steve Capper <steve.cap...@arm.com>
Signed-off-by: Will Deacon <will.dea...@arm.com>
---
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
