tcp_htcp: last_cong bug fix

This patch fixes a minor bug in tcp_htcp.c which has been
highlighted by Lachlan Andrew and Lawrence Stewart. Currently, the
time since the last congestion event, which is stored in variable
last_cong, is reset whenever there is a state change into
TCP_CA_Open. This includes transitions of the type
TCP_CA_Open->TCP_CA_Disorder->TCP_CA_Open which are not associated
with backoff of cwnd. The patch changes last_cong to be updated
only on transitions into TCP_CA_Open that occur after experiencing
the congestion-related states TCP_CA_Loss, TCP_CA_Recovery,
TCP_CA_CWR.

Signed-off-by: Doug Leith <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

net/ipv4/tcp_htcp.c

···
 	const struct tcp_sock *tp = tcp_sk(sk);
 	struct htcp *ca = inet_csk_ca(sk);
 
-	ca->last_cong = ca->undo_last_cong;
-	ca->maxRTT = ca->undo_maxRTT;
-	ca->old_maxB = ca->undo_old_maxB;
+	if (ca->undo_last_cong) {
+		ca->last_cong = ca->undo_last_cong;
+		ca->maxRTT = ca->undo_maxRTT;
+		ca->old_maxB = ca->undo_old_maxB;
+		ca->undo_last_cong = 0;
+	}
 
 	return max(tp->snd_cwnd, (tp->snd_ssthresh << 7) / ca->beta);
 }
···
 	case TCP_CA_Open:
 		{
 			struct htcp *ca = inet_csk_ca(sk);
-			ca->last_cong = jiffies;
+			if (ca->undo_last_cong) {
+				ca->last_cong = jiffies;
+				ca->undo_last_cong = 0;
+			}
 		}
 		break;
 	case TCP_CA_CWR: