[TCP] tcp_highspeed: Fix AI updates.

I think there is still a problem with the AIMD parameter update in
HighSpeed TCP code.

Line 125~138 of the code (net/ipv4/tcp_highspeed.c):

	/* Update AIMD parameters */
	if (tp->snd_cwnd > hstcp_aimd_vals[ca->ai].cwnd) {
		while (tp->snd_cwnd > hstcp_aimd_vals[ca->ai].cwnd &&
		       ca->ai < HSTCP_AIMD_MAX - 1)
			ca->ai++;
	} else if (tp->snd_cwnd < hstcp_aimd_vals[ca->ai].cwnd) {
		while (tp->snd_cwnd > hstcp_aimd_vals[ca->ai].cwnd &&
		       ca->ai > 0)
			ca->ai--;

In fact, the second part (decreasing ca->ai) never decreases since the
while loop's inequality is in the reverse direction. This leads to
unfairness with multiple flows (once a flow happens to enjoy a higher
ca->ai, it keeps enjoying that even its cwnd decreases)

Here is a tentative fix (I also added a comment, trying to keep the
change clear):

Acked-by: Stephen Hemminger <shemminger@osdl.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/tcp_highspeed.c

@@ -139,14 +139,19 @@ static void hstcp_cong_avoid(struct sock *sk, u32 adk, u32 rtt,
 				tp->snd_cwnd++;
 		}
 	} else {
-		/* Update AIMD parameters */
+		/* Update AIMD parameters.
+		 *
+		 * We want to guarantee that:
+		 *     hstcp_aimd_vals[ca->ai-1].cwnd <
+		 *     snd_cwnd <=
+		 *     hstcp_aimd_vals[ca->ai].cwnd
+		 */
 		if (tp->snd_cwnd > hstcp_aimd_vals[ca->ai].cwnd) {
 			while (tp->snd_cwnd > hstcp_aimd_vals[ca->ai].cwnd &&
 			       ca->ai < HSTCP_AIMD_MAX - 1)
 				ca->ai++;
-		} else if (tp->snd_cwnd < hstcp_aimd_vals[ca->ai].cwnd) {
-			while (tp->snd_cwnd > hstcp_aimd_vals[ca->ai].cwnd &&
-			       ca->ai > 0)
+		} else if (ca->ai && tp->snd_cwnd <= hstcp_aimd_vals[ca->ai-1].cwnd) {
+			while (ca->ai && tp->snd_cwnd <= hstcp_aimd_vals[ca->ai-1].cwnd)
 				ca->ai--;
 		}