[PATCH 5.6 037/106] tipc: fix incorrect increasing of link window

[Greg Kroah-Hartman]

From: Tuong Lien <tuong.t.lien@xxxxxxxxxxxxxx>

commit edadedf1c5b4e4404192a0a4c3c0c05e3b7672ab upstream.

In commit 16ad3f4022bb ("tipc: introduce variable window congestion
control"), we allow link window to change with the congestion avoidance
algorithm. However, there is a bug that during the slow-start if packet
retransmission occurs, the link will enter the fast-recovery phase, set
its window to the 'ssthresh' which is never less than 300, so the link
window suddenly increases to that limit instead of decreasing.

Consequently, two issues have been observed:

- For broadcast-link: it can leave a gap between the link queues that a
new packet will be inserted and sent before the previous ones, i.e. not
in-order.

- For unicast: the algorithm does not work as expected, the link window
jumps to the slow-start threshold whereas packet retransmission occurs.

This commit fixes the issues by avoiding such the link window increase,
but still decreasing if the 'ssthresh' is lowered.

Fixes: 16ad3f4022bb ("tipc: introduce variable window congestion control")
Acked-by: Jon Maloy <jmaloy@xxxxxxxxxx>
Signed-off-by: Tuong Lien <tuong.t.lien@xxxxxxxxxxxxxx>
Signed-off-by: David S. Miller <davem@xxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 net/tipc/link.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/net/tipc/link.c
+++ b/net/tipc/link.c
@@ -1065,7 +1065,7 @@ static void tipc_link_update_cwin(struct
 	/* Enter fast recovery */
 	if (unlikely(retransmitted)) {
 		l->ssthresh = max_t(u16, l->window / 2, 300);
-		l->window = l->ssthresh;
+		l->window = min_t(u16, l->ssthresh, l->window);
 		return;
 	}
 	/* Enter slow start */