tipc: improve poll() for group member socket

The current criteria for returning POLLOUT from a group member socket is
too simplistic. It basically returns POLLOUT as soon as the group has
external destinations, something obviously leading to a lot of spinning
during destination congestion situations. At the same time, the internal
congestion handling is unnecessarily complex.

We now change this as follows.

- We introduce an 'open' flag in  struct tipc_group. This flag is used
  only to help poll() get the setting of POLLOUT right, and *not* for
  congeston handling as such. This means that a user can choose to
  ignore an  EAGAIN for a destination and go on sending messages to
  other destinations in the group if he wants to.

- The flag is set to false every time we return EAGAIN on a send call.

- The flag is set to true every time any member, i.e., not necessarily
  the member that caused EAGAIN, is removed from the small_win list.

- We remove the group member 'usr_pending' flag. The size of the send
  window and presence in the 'small_win' list is sufficient criteria
  for recognizing congestion.

This solution seems to be a reasonable compromise between 'anycast',
which is normally not waiting for POLLOUT for a specific destination,
and the other three send modes, which are.

Acked-by: Ying Xue <ying.xue@windriver.com>
Signed-off-by: Jon Maloy <jon.maloy@ericsson.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/tipc/socket.c

@@ -715,7 +715,7 @@ static unsigned int tipc_poll(struct file *file, struct socket *sock,
 {
 	struct sock *sk = sock->sk;
 	struct tipc_sock *tsk = tipc_sk(sk);
-	struct tipc_group *grp = tsk->group;
+	struct tipc_group *grp;
 	u32 revents = 0;
 
 	sock_poll_wait(file, sk_sleep(sk), wait);
@@ -736,9 +736,9 @@ static unsigned int tipc_poll(struct file *file, struct socket *sock,
 			revents |= POLLIN | POLLRDNORM;
 		break;
 	case TIPC_OPEN:
-		if (!grp || tipc_group_size(grp))
-			if (!tsk->cong_link_cnt)
-				revents |= POLLOUT;
+		grp = tsk->group;
+		if ((!grp || tipc_group_is_open(grp)) && !tsk->cong_link_cnt)
+			revents |= POLLOUT;
 		if (!tipc_sk_type_connectionless(sk))
 			break;
 		if (skb_queue_empty(&sk->sk_receive_queue))