path: root/net/ipv4/tcp_output.c
diff options
authorYuchung Cheng <ycheng@google.com>2018-07-18 13:56:35 -0700
committerDavid S. Miller <davem@davemloft.net>2018-07-20 14:32:23 -0700
commit27cde44a259c380a3c09066fc4b42de7dde9b1ad (patch)
tree2f0249f49428f487fcda2249b5dab4d82dd8e514 /net/ipv4/tcp_output.c
parenttcp: helpers to send special DCTCP ack (diff)
tcp: do not cancel delay-AcK on DCTCP special ACK
Currently when a DCTCP receiver delays an ACK and receive a data packet with a different CE mark from the previous one's, it sends two immediate ACKs acking previous and latest sequences respectly (for ECN accounting). Previously sending the first ACK may mark off the delayed ACK timer (tcp_event_ack_sent). This may subsequently prevent sending the second ACK to acknowledge the latest sequence (tcp_ack_snd_check). The culprit is that tcp_send_ack() assumes it always acknowleges the latest sequence, which is not true for the first special ACK. The fix is to not make the assumption in tcp_send_ack and check the actual ack sequence before cancelling the delayed ACK. Further it's safer to pass the ack sequence number as a local variable into tcp_send_ack routine, instead of intercepting tp->rcv_nxt to avoid future bugs like this. Reported-by: Neal Cardwell <ncardwell@google.com> Signed-off-by: Yuchung Cheng <ycheng@google.com> Acked-by: Neal Cardwell <ncardwell@google.com> Signed-off-by: Eric Dumazet <edumazet@google.com> Signed-off-by: David S. Miller <davem@davemloft.net>
Diffstat (limited to 'net/ipv4/tcp_output.c')
1 files changed, 7 insertions, 3 deletions
diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index ee1b0705321d..c4172c1fb198 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -160,7 +160,8 @@ static void tcp_event_data_sent(struct tcp_sock *tp,
/* Account for an ACK we sent. */
-static inline void tcp_event_ack_sent(struct sock *sk, unsigned int pkts)
+static inline void tcp_event_ack_sent(struct sock *sk, unsigned int pkts,
+ u32 rcv_nxt)
struct tcp_sock *tp = tcp_sk(sk);
@@ -171,6 +172,9 @@ static inline void tcp_event_ack_sent(struct sock *sk, unsigned int pkts)
if (hrtimer_try_to_cancel(&tp->compressed_ack_timer) == 1)
+ if (unlikely(rcv_nxt != tp->rcv_nxt))
+ return; /* Special ACK sent by DCTCP to reflect ECN */
tcp_dec_quickack_mode(sk, pkts);
inet_csk_clear_xmit_timer(sk, ICSK_TIME_DACK);
@@ -1141,7 +1145,7 @@ static int __tcp_transmit_skb(struct sock *sk, struct sk_buff *skb,
icsk->icsk_af_ops->send_check(sk, skb);
if (likely(tcb->tcp_flags & TCPHDR_ACK))
- tcp_event_ack_sent(sk, tcp_skb_pcount(skb));
+ tcp_event_ack_sent(sk, tcp_skb_pcount(skb), rcv_nxt);
if (skb->len != tcp_header_size) {
tcp_event_data_sent(tp, sk);
@@ -3613,12 +3617,12 @@ void __tcp_send_ack(struct sock *sk, u32 rcv_nxt)
/* Send it off, this clears delayed acks for us. */
__tcp_transmit_skb(sk, buff, 0, (__force gfp_t)0, rcv_nxt);
void tcp_send_ack(struct sock *sk)
__tcp_send_ack(sk, tcp_sk(sk)->rcv_nxt);
/* This routine sends a packet with an out of date sequence
* number. It assumes the other end will try to ack it.