tcp: simplify window probe aborting on USER_TIMEOUT

Previously we use the next unsent skb's timestamp to determine
when to abort a socket stalling on window probes. This no longer
works as skb timestamp reflects the last instead of the first
transmission.

Instead we can estimate how long the socket has been stalling
with the probe count and the exponential backoff behavior.

Signed-off-by: Yuchung Cheng <ycheng@google.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Neal Cardwell <ncardwell@google.com>
Reviewed-by: Soheil Hassas Yeganeh <soheil@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/tcp_timer.c

@@ -333,7 +333,6 @@ static void tcp_probe_timer(struct sock *sk)
 	struct sk_buff *skb = tcp_send_head(sk);
 	struct tcp_sock *tp = tcp_sk(sk);
 	int max_probes;
-	u32 start_ts;
 
 	if (tp->packets_out || !skb) {
 		icsk->icsk_probes_out = 0;
@@ -348,12 +347,13 @@ static void tcp_probe_timer(struct sock *sk)
 	 * corresponding system limit. We also implement similar policy when
 	 * we use RTO to probe window in tcp_retransmit_timer().
 	 */
-	start_ts = tcp_skb_timestamp(skb);
-	if (!start_ts)
-		skb->skb_mstamp_ns = tp->tcp_clock_cache;
-	else if (icsk->icsk_user_timeout &&
-		 (s32)(tcp_time_stamp(tp) - start_ts) > icsk->icsk_user_timeout)
-		goto abort;
+	if (icsk->icsk_user_timeout) {
+		u32 elapsed = tcp_model_timeout(sk, icsk->icsk_probes_out,
+						tcp_probe0_base(sk));
+
+		if (elapsed >= icsk->icsk_user_timeout)
+			goto abort;
+	}
 
 	max_probes = sock_net(sk)->ipv4.sysctl_tcp_retries2;
 	if (sock_flag(sk, SOCK_DEAD)) {