rxrpc: Make service connection lookup always check for retry

When an RxRPC service packet comes in, the target connection is looked up
by an rb-tree search under RCU and a read-locked seqlock; the seqlock retry
check is, however, currently skipped if we got a match, but probably
shouldn't be in case the connection we found gets replaced whilst we're
doing a search.

Make the lookup procedure always go through need_seqretry(), even if the
lookup was successful.  This makes sure we always pick up on a write-lock
event.

On the other hand, since we don't take a ref on the object, but rely on RCU
to prevent its destruction after dropping the seqlock, I'm not sure this is
necessary.

Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/rxrpc/conn_service.c

@@ -50,12 +50,11 @@ struct rxrpc_connection *rxrpc_find_service_conn_rcu(struct rxrpc_peer *peer,
 			else if (conn->proto.index_key > k.index_key)
 				p = rcu_dereference_raw(p->rb_right);
 			else
-				goto done;
+				break;
 			conn = NULL;
 		}
 	} while (need_seqretry(&peer->service_conn_lock, seq));
 
-done:
 	done_seqretry(&peer->service_conn_lock, seq);
 	_leave(" = %d", conn ? conn->debug_id : -1);
 	return conn;