rds: MP-RDS may use an invalid c_path

rds_sendmsg() calls rds_send_mprds_hash() to find a c_path to use to
send a message.  Suppose the RDS connection is not yet up.  In
rds_send_mprds_hash(), it does

	if (conn->c_npaths == 0)
		wait_event_interruptible(conn->c_hs_waitq,
					 (conn->c_npaths != 0));

If it is interrupted before the connection is set up,
rds_send_mprds_hash() will return a non-zero hash value.  Hence
rds_sendmsg() will use a non-zero c_path to send the message.  But if
the RDS connection ends up to be non-MP capable, the message will be
lost as only the zero c_path can be used.
Signed-off-by: Ka-Cheong Poon <ka-cheong.poon@oracle.com>
Acked-by: Santosh Shilimkar <santosh.shilimkar@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/rds/send.c

 /*
- * Copyright (c) 2006 Oracle.  All rights reserved.
+ * Copyright (c) 2006, 2018 Oracle and/or its affiliates. All rights reserved.
  *
  * This software is available to you under a choice of one of two
  * licenses.  You may choose to be licensed under the terms of the GNU
@@ -1017,10 +1017,15 @@ static int rds_send_mprds_hash(struct rds_sock *rs, struct rds_connection *conn)
 	if (conn->c_npaths == 0 && hash != 0) {
 		rds_send_ping(conn, 0);
 
-		if (conn->c_npaths == 0) {
-			wait_event_interruptible(conn->c_hs_waitq,
-						 (conn->c_npaths != 0));
-		}
+		/* The underlying connection is not up yet.  Need to wait
+		 * until it is up to be sure that the non-zero c_path can be
+		 * used.  But if we are interrupted, we have to use the zero
+		 * c_path in case the connection ends up being non-MP capable.
+		 */
+		if (conn->c_npaths == 0)
+			if (wait_event_interruptible(conn->c_hs_waitq,
+						     conn->c_npaths != 0))
+				hash = 0;
 		if (conn->c_npaths == 1)
 			hash = 0;
 	}