Merge branch 'master' into upstream-fixes

1 files changed, 11 insertions, 4 deletions

diff --git a/kernel/sys.c b/kernel/sys.c
index c7675c1bfdf2..6e2101dec0fc 100644
--- a/kernel/sys.c
+++ b/kernel/sys.c
@@ -323,11 +323,18 @@ EXPORT_SYMBOL_GPL(blocking_notifier_chain_unregister);
 int blocking_notifier_call_chain(struct blocking_notifier_head *nh,
                 unsigned long val, void *v)
 {
-        int ret;
+        int ret = NOTIFY_DONE;
 
-        down_read(&nh->rwsem);
-        ret = notifier_call_chain(&nh->head, val, v);
-        up_read(&nh->rwsem);
+        /*
+         * We check the head outside the lock, but if this access is
+         * racy then it does not matter what the result of the test
+         * is, we re-check the list after having taken the lock anyway:
+         */
+        if (rcu_dereference(nh->head)) {
+                down_read(&nh->rwsem);
+                ret = notifier_call_chain(&nh->head, val, v);
+                up_read(&nh->rwsem);
+        }
         return ret;
 }