X-Git-Url: https://err.no/cgi-bin/gitweb.cgi?a=blobdiff_plain;f=kernel%2Fsys.c;h=6e2101dec0fcfc9e95c52a1ac53bc6898b976edf;hb=d5112a4f31a361409d3c57dc9d58dd69f8014bef;hp=c7675c1bfdf24e8c9d45b22e67da5c5679ba662b;hpb=9cdf083f981b8d37b3212400a359368661385099;p=linux-2.6

diff --git a/kernel/sys.c b/kernel/sys.c
index c7675c1bfd..6e2101dec0 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;
 }