netdev - [PATCH net-next 2/3] tcp: consider using standard rtx logic in tcp_rcv_fastopen

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Message-Id: <20210311203506.3450792-3-eric.dumazet@gmail.com>
Date:   Thu, 11 Mar 2021 12:35:05 -0800
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>
Cc:     netdev <netdev@...r.kernel.org>,
        Eric Dumazet <edumazet@...gle.com>,
        Eric Dumazet <eric.dumazet@...il.com>,
        Neal Cardwell <ncardwell@...gle.com>,
        Yuchung Cheng <ycheng@...gle.com>,
        Neil Spring <ntspring@...com>
Subject: [PATCH net-next 2/3] tcp: consider using standard rtx logic in tcp_rcv_fastopen_synack()

From: Eric Dumazet <edumazet@...gle.com>

Jakub reported Data included in a Fastopen SYN that had to be
retransmit would have to wait for an RTO if TX completions are slow,
even with prior fix.

This is because tcp_rcv_fastopen_synack() does not use standard
rtx logic, meaning TSQ handler exits early in tcp_tsq_write()
because tp->lost_out == tp->retrans_out

Lets make tcp_rcv_fastopen_synack() use standard rtx logic,
by using tcp_mark_skb_lost() on the skb thats needs to be
sent again.

Not this raised a warning in tcp_fastretrans_alert() during my tests
since we consider the data not being aknowledged
by the receiver does not mean packet was lost on the network.

Signed-off-by: Eric Dumazet <edumazet@...gle.com>
Reported-by: Jakub Kicinski <kuba@...nel.org>
Cc: Neal Cardwell <ncardwell@...gle.com>
Cc: Yuchung Cheng <ycheng@...gle.com>
---
 net/ipv4/tcp_input.c | 10 ++++------
 1 file changed, 4 insertions(+), 6 deletions(-)

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 69a545db80d2ead47ffcf2f3819a6d066e95f35d..4cf4dd532d1c65bba417a66ba6b7783491b6380a 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -2914,7 +2914,7 @@ static void tcp_fastretrans_alert(struct sock *sk, const u32 prior_snd_una,
 	/* D. Check state exit conditions. State can be terminated
 	 *    when high_seq is ACKed. */
 	if (icsk->icsk_ca_state == TCP_CA_Open) {
-		WARN_ON(tp->retrans_out != 0);
+		WARN_ON(tp->retrans_out != 0 && !tp->syn_data);
 		tp->retrans_stamp = 0;
 	} else if (!before(tp->snd_una, tp->high_seq)) {
 		switch (icsk->icsk_ca_state) {
@@ -5994,11 +5994,9 @@ static bool tcp_rcv_fastopen_synack(struct sock *sk, struct sk_buff *synack,
 			tp->fastopen_client_fail = TFO_SYN_RETRANSMITTED;
 		else
 			tp->fastopen_client_fail = TFO_DATA_NOT_ACKED;
-		skb_rbtree_walk_from(data) {
-			if (__tcp_retransmit_skb(sk, data, 1))
-				break;
-		}
-		tcp_rearm_rto(sk);
+		skb_rbtree_walk_from(data)
+			 tcp_mark_skb_lost(sk, data);
+		tcp_xmit_retransmit_queue(sk);
 		NET_INC_STATS(sock_net(sk),
 				LINUX_MIB_TCPFASTOPENACTIVEFAIL);
 		return true;
-- 
2.31.0.rc2.261.g7f71774620-goog