[PATCH 4.4] memfd: Use radix_tree_deref_slot_protected to avoid the warning.

[zhong jiang <zhongjiang@huawei.com>]

The commit eb4058d8daf8 ("memfd: Fix locking when tagging pins")
introduces the following warning messages.

*WARNING: suspicious RCU usage in memfd_wait_for_pins*

It is because we still use radix_tree_deref_slot without read_rcu_lock.
We should use radix_tree_deref_slot_protected instead in the case.

Cc: stable@vger.kernel.org
Fixes: eb4058d8daf8 ("memfd: Fix locking when tagging pins")
Signed-off-by: zhong jiang <zhongjiang@huawei.com>
---
 mm/shmem.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/shmem.c b/mm/shmem.c
index 62668379623b..e40239bf6dfe 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -1862,7 +1862,7 @@ static void shmem_tag_pins(struct address_space *mapping)
 	spin_lock_irq(&mapping->tree_lock);
 restart:
 	radix_tree_for_each_slot(slot, &mapping->page_tree, &iter, start) {
-		page = radix_tree_deref_slot(slot);
+		page = radix_tree_deref_slot_protected(slot, &mapping->tree_lock);
 		if (!page || radix_tree_exception(page)) {
 			if (radix_tree_deref_retry(page))
 				goto restart;
-- 
2.20.1