libceph: move ceph_con_send() closed check under the con mutex

Take the con mutex before checking whether the connection is closed to
avoid racing with someone else closing it.
Signed-off-by: Sage Weil <sage@inktank.com>

net/ceph/messenger.c

@@ -2453,22 +2453,20 @@ static void clear_standby(struct ceph_connection *con)
  */
 void ceph_con_send(struct ceph_connection *con, struct ceph_msg *msg)
 {
-	if (test_bit(CLOSED, &con->state)) {
-		dout("con_send %p closed, dropping %p\n", con, msg);
-		ceph_msg_put(msg);
-		return;
-	}
-
 	/* set src+dst */
 	msg->hdr.src = con->msgr->inst.name;
-
 	BUG_ON(msg->front.iov_len != le32_to_cpu(msg->hdr.front_len));
-
 	msg->needs_out_seq = true;
 
-	/* queue */
 	mutex_lock(&con->mutex);
 
+	if (test_bit(CLOSED, &con->state)) {
+		dout("con_send %p closed, dropping %p\n", con, msg);
+		ceph_msg_put(msg);
+		mutex_unlock(&con->mutex);
+		return;
+	}
+
 	BUG_ON(msg->con != NULL);
 	msg->con = con->ops->get(con);
 	BUG_ON(msg->con == NULL);