drbd: More random to the connect logic

Since the listening socket is open all the time, it was possible to
get into stable "initial packet S crossed" loops.

* when both sides realize in the drbd_socket_okay() call at the end
  of the loop that the other side closed the main socket you had
  the chance to get into a stable loop with repeated "packet S crossed"
  messages.

* when both sides do not realize with the drbd_socket_okay() call at the end
  of the loop that the other side closed the main socket you had
  the chance to get into a stable loop with alternating "packet S crossed"
  "packet M crossed" messages.

In order to break out these stable loops randomize the behaviour if
such a crossing of P_INITIAL_DATA or P_INITIAL_META packets is detected.
Signed-off-by: Philipp Reisner <philipp.reisner@linbit.com>
Signed-off-by: Lars Ellenberg <lars.ellenberg@linbit.com>

drivers/block/drbd/drbd_receiver.c

@@ -949,20 +949,25 @@ static int conn_connect(struct drbd_tconn *tconn)
 				if (sock.socket) {
 					conn_warn(tconn, "initial packet S crossed\n");
 					sock_release(sock.socket);
+					sock.socket = s;
+					goto randomize;
 				}
 				sock.socket = s;
 				break;
 			case P_INITIAL_META:
+				set_bit(DISCARD_CONCURRENT, &tconn->flags);
 				if (msock.socket) {
 					conn_warn(tconn, "initial packet M crossed\n");
 					sock_release(msock.socket);
+					msock.socket = s;
+					goto randomize;
 				}
 				msock.socket = s;
-				set_bit(DISCARD_CONCURRENT, &tconn->flags);
 				break;
 			default:
 				conn_warn(tconn, "Error receiving initial packet\n");
 				sock_release(s);
+randomize:
 				if (random32() & 1)
 					goto retry;
 			}