[PATCH net-next 2/2] tcp: TLP retransmits last if failed to send new packet

[Yuchung Cheng <ycheng@google.com>]

When TLP fails to send new packet because of receive window
limit or other reasons, it should fall back to retransmit
the last packet, instead of resorting to RTO.

Signed-off-by: Yuchung Cheng <ycheng@google.com>
Signed-off-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: Nanditad Dukkipati <nanditad@google.com>
---
 net/ipv4/tcp_output.c | 14 +++++++++-----
 1 file changed, 9 insertions(+), 5 deletions(-)

diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index deb944b..4f238bb 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -2226,7 +2226,7 @@ static bool skb_still_in_host_queue(const struct sock *sk,
 	return false;
 }
 
-/* When probe timeout (PTO) fires, send a new segment if one exists, else
+/* When probe timeout (PTO) fires, try send a new segment if possible, else
  * retransmit the last segment.
  */
 void tcp_send_loss_probe(struct sock *sk)
@@ -2237,9 +2237,14 @@ void tcp_send_loss_probe(struct sock *sk)
 	int mss = tcp_current_mss(sk);
 	int err = -1;
 
-	if (tcp_send_head(sk)) {
+	skb = tcp_send_head(sk);
+	if (skb) {
 		err = tcp_write_xmit(sk, mss, TCP_NAGLE_OFF, 2, GFP_ATOMIC);
-		goto rearm_timer;
+		if (!err)
+			goto rearm_timer;
+		skb = tcp_write_queue_prev(sk, skb);
+	} else {
+		skb = tcp_write_queue_tail(sk);
 	}
 
 	/* At most one outstanding TLP retransmission. */
@@ -2247,7 +2252,6 @@ void tcp_send_loss_probe(struct sock *sk)
 		goto rearm_timer;
 
 	/* Retransmit last segment. */
-	skb = tcp_write_queue_tail(sk);
 	if (WARN_ON(!skb))
 		goto rearm_timer;
 
@@ -2262,7 +2266,7 @@ void tcp_send_loss_probe(struct sock *sk)
 		if (unlikely(tcp_fragment(sk, skb, (pcount - 1) * mss, mss,
 					  GFP_ATOMIC)))
 			goto rearm_timer;
-		skb = tcp_write_queue_tail(sk);
+		skb = tcp_write_queue_next(sk, skb);
 	}
 
 	if (WARN_ON(!skb || !tcp_skb_pcount(skb)))
-- 
2.4.3.573.g4eafbef