[PATCH] Race against parent deletion in key_user_lookup()

I looked at some of the oops reports against keyrings, I think the problem
is that the search isn't restarted after dropping the key_user_lock, *p
will still be NULL when we get back to try_again and look through the tree.

It looks like the intention was that the search start over from scratch.
Signed-off-by: Alexander Nyberg <alexn@dsv.su.se>
Cc: David Howells <dhowells@redhat.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

security/keys/key.c

@@ -57,9 +57,10 @@ struct key_user *key_user_lookup(uid_t uid)
 {
 	struct key_user *candidate = NULL, *user;
 	struct rb_node *parent = NULL;
-	struct rb_node **p = &key_user_tree.rb_node;
+	struct rb_node **p;
 
  try_again:
+	p = &key_user_tree.rb_node;
 	spin_lock(&key_user_lock);
 
 	/* search the tree for a user record with a matching UID */