Merge branch 'for_paulus' of master.kernel.org:/pub/scm/linux/kernel/git/galak/powerpc into for-2.6.21

1 files changed, 11 insertions, 4 deletions

diff --git a/kernel/sys.c b/kernel/sys.c
index c7675c1bfdf..6e2101dec0f 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;
 }