In we set the old _refcount value after verifying that the old value was
indeed 0.
VM_BUG_ON_PAGE(page_count(page) != 0, page);
< the _refcount may change here>
atomic_set_release(&page->_refcount, count);
To avoid the smal gap where _refcount may change lets verify the time
of the _refcount at the time of the set operation.
Use atomic_xchg_release() and at the set time verify that the value
was 0.
Signed-off-by: Pasha Tatashin <pasha.tatashin@xxxxxxxxxx>
---
include/linux/page_ref.h | 5 ++---
1 file changed, 2 insertions(+), 3 deletions(-)
diff --git a/include/linux/page_ref.h b/include/linux/page_ref.h
index 243fc60ae6c8..9efabeff4e06 100644
--- a/include/linux/page_ref.h
+++ b/include/linux/page_ref.h
@@ -322,10 +322,9 @@ static inline int folio_ref_freeze(struct folio *folio, int count)
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);
+ int old_val = atomic_xchg_release(&page->_refcount, count);
- atomic_set_release(&page->_refcount, count);
+ VM_BUG_ON_PAGE(count == 0 || old_val != 0, page);
if (page_ref_tracepoint_active(page_ref_unfreeze))
__page_ref_unfreeze(page, count);
}
--
2.34.1.307.g9b7440fafd-goog
