net-tcp_bbr: set tp->snd_ssthresh to BDP upon STARTUP exit

Set tp->snd_ssthresh to BDP upon STARTUP exit. This allows us
to check if a BBR flow exited STARTUP and the BDP at the
time of STARTUP exit with SCM_TIMESTAMPING_OPT_STATS. Since BBR does not
use snd_ssthresh this fix has no impact on BBR's behavior.

Signed-off-by: Yousuk Seung <ysseung@google.com>
Signed-off-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: Priyaranjan Jha <priyarjha@google.com>
Signed-off-by: Soheil Hassas Yeganeh <soheil@google.com>
Signed-off-by: Yuchung Cheng <ycheng@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/tcp_bbr.c

@@ -731,6 +731,8 @@ static void bbr_check_drain(struct sock *sk, const struct rate_sample *rs)
 		bbr->mode = BBR_DRAIN;	/* drain queue we created */
 		bbr->pacing_gain = bbr_drain_gain;	/* pace slow to drain */
 		bbr->cwnd_gain = bbr_high_gain;	/* maintain cwnd */
+		tcp_sk(sk)->snd_ssthresh =
+				bbr_target_cwnd(sk, bbr_max_bw(sk), BBR_UNIT);
 	}	/* fall through to check if in-flight is already small: */
 	if (bbr->mode == BBR_DRAIN &&
 	    tcp_packets_in_flight(tcp_sk(sk)) <=
@@ -834,6 +836,7 @@ static void bbr_init(struct sock *sk)
 	struct bbr *bbr = inet_csk_ca(sk);
 
 	bbr->prior_cwnd = 0;
+	tp->snd_ssthresh = TCP_INFINITE_SSTHRESH;
 	bbr->rtt_cnt = 0;
 	bbr->next_rtt_delivered = 0;
 	bbr->prev_ca_state = TCP_CA_Open;
@@ -886,7 +889,7 @@ static u32 bbr_undo_cwnd(struct sock *sk)
 static u32 bbr_ssthresh(struct sock *sk)
 {
 	bbr_save_cwnd(sk);
-	return TCP_INFINITE_SSTHRESH;	 /* BBR does not use ssthresh */
+	return tcp_sk(sk)->snd_ssthresh;
 }
 
 static size_t bbr_get_info(struct sock *sk, u32 ext, int *attr,