tipc: extend broadcast link window size

The default fix broadcast window size is currently set to 20 packets.
This is a very low value, set at a time when we were still testing on
10 Mb/s hubs, and a change to it is long overdue.

Commit 7845989c ("net: tipc: fix stall during bclink wakeup procedure")
revealed a problem with this low value. For messages of importance LOW,
the backlog queue limit will be  calculated to 30 packets, while a
single, maximum sized message of 66000 bytes, carried across a 1500 MTU
network consists of 46 packets.

This leads to the following scenario (among others leading to the same
situation):

1: Msg 1 of 46 packets is sent. 20 packets go to the transmit queue, 26
   packets to the backlog queue.
2: Msg 2 of 46 packets is attempted sent, but rejected because there is
   no more space in the backlog queue at this level. The sender is added
   to the wakeup queue with a "pending packets chain size" number of 46.
3: Some packets in the transmit queue are acked and released. We try to
   wake up the sender, but the pending size of 46 is bigger than the LOW
   wakeup limit of 30, so this doesn't happen.
5: Subsequent acks releases all the remaining buffers. Each time we test
   for the wakeup criteria and find that 46 still is larger than 30,
   even after both the transmit and the backlog queues are empty.
6: The sender is never woken up and given a chance to send its message.
   He is stuck.

We could now loosen the wakeup criteria (used by link_prepare_wakeup())
to become equal to the send criteria (used by tipc_link_xmit()), i.e.,
by ignoring the "pending packets chain size" value altogether, or we can
just increase the queue limits so that the criteria can be satisfied
anyway. There are good reasons (potentially multiple waiting senders) to
not opt for the former solution, so we choose the latter one.

This commit fixes the problem by giving the broadcast link window a
default value of 50 packets. We also introduce a new minimum link
window size BCLINK_MIN_WIN of 32, which is enough to always avoid the
described situation. Finally, in order to not break any existing users
which may set the window explicitly, we enforce that the window is set
to the new minimum value in case the user is trying to set it to
anything lower.

Fixes: 7845989c ("net: tipc: fix stall during bclink wakeup procedure")
Signed-off-by: Jon Maloy <jon.maloy@ericsson.com>
Reviewed-by: Ying Xue <ying.xue@windriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/tipc/bcast.c

@@ -42,7 +42,8 @@
 #include "core.h"
 
 #define	MAX_PKT_DEFAULT_MCAST	1500	/* bcast link max packet size (fixed) */
-#define	BCLINK_WIN_DEFAULT	20	/* bcast link window size (default) */
+#define	BCLINK_WIN_DEFAULT	50	/* bcast link window size (default) */
+#define	BCLINK_WIN_MIN	        32	/* bcast minimum link window size */
 
 const char tipc_bclink_name[] = "broadcast-link";
 
@@ -908,9 +909,10 @@ int tipc_bclink_set_queue_limits(struct net *net, u32 limit)
 
 	if (!bcl)
 		return -ENOPROTOOPT;
-	if ((limit < TIPC_MIN_LINK_WIN) || (limit > TIPC_MAX_LINK_WIN))
+	if (limit < BCLINK_WIN_MIN)
+		limit = BCLINK_WIN_MIN;
+	if (limit > TIPC_MAX_LINK_WIN)
 		return -EINVAL;
-
 	tipc_bclink_lock(net);
 	tipc_link_set_queue_limits(bcl, limit);
 	tipc_bclink_unlock(net);