tcp_bbr: in restart from idle, see if we should exit PROBE_RTT

This patch fix the case where BBR does not exit PROBE_RTT mode when
it restarts from idle. When BBR restarts from idle and if BBR is in
PROBE_RTT mode, BBR should check if it's time to exit PROBE_RTT. If
yes, then BBR should exit PROBE_RTT mode and restore the cwnd to its
full value.

Fixes: 0f8782ea ("tcp_bbr: add BBR congestion control")
Signed-off-by: Kevin Yang <yyd@google.com>
Signed-off-by: Neal Cardwell <ncardwell@google.com>
Reviewed-by: Yuchung Cheng <ycheng@google.com>
Reviewed-by: Soheil Hassas Yeganeh <soheil@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/tcp_bbr.c

@@ -174,6 +174,8 @@ static const u32 bbr_lt_bw_diff = 4000 / 8;
 /* If we estimate we're policed, use lt_bw for this many round trips: */
 static const u32 bbr_lt_bw_max_rtts = 48;
 
+static void bbr_check_probe_rtt_done(struct sock *sk);
+
 /* Do we estimate that STARTUP filled the pipe? */
 static bool bbr_full_bw_reached(const struct sock *sk)
 {
@@ -308,6 +310,8 @@ static void bbr_cwnd_event(struct sock *sk, enum tcp_ca_event event)
 		 */
 		if (bbr->mode == BBR_PROBE_BW)
 			bbr_set_pacing_rate(sk, bbr_bw(sk), BBR_UNIT);
+		else if (bbr->mode == BBR_PROBE_RTT)
+			bbr_check_probe_rtt_done(sk);
 	}
 }