Patch "tcp: fix mishandling when the sack compression is deferred." has been added to the 6.1-stable tree

[Sasha Levin <sashal@xxxxxxxxxx>]

This is a note to let you know that I've just added the patch titled

    tcp: fix mishandling when the sack compression is deferred.

to the 6.1-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     tcp-fix-mishandling-when-the-sack-compression-is-def.patch
and it can be found in the queue-6.1 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.



commit fb98adcac992729b54a2ef14a89079824769511f
Author: fuyuanli <fuyuanli@xxxxxxxxxxxxxx>
Date:   Wed May 31 16:01:50 2023 +0800

    tcp: fix mishandling when the sack compression is deferred.
    
    [ Upstream commit 30c6f0bf9579debce27e45fac34fdc97e46acacc ]
    
    In this patch, we mainly try to handle sending a compressed ack
    correctly if it's deferred.
    
    Here are more details in the old logic:
    When sack compression is triggered in the tcp_compressed_ack_kick(),
    if the sock is owned by user, it will set TCP_DELACK_TIMER_DEFERRED
    and then defer to the release cb phrase. Later once user releases
    the sock, tcp_delack_timer_handler() should send a ack as expected,
    which, however, cannot happen due to lack of ICSK_ACK_TIMER flag.
    Therefore, the receiver would not sent an ack until the sender's
    retransmission timeout. It definitely increases unnecessary latency.
    
    Fixes: 5d9f4262b7ea ("tcp: add SACK compression")
    Suggested-by: Eric Dumazet <edumazet@xxxxxxxxxx>
    Signed-off-by: fuyuanli <fuyuanli@xxxxxxxxxxxxxx>
    Signed-off-by: Jason Xing <kerneljasonxing@xxxxxxxxx>
    Link: https://lore.kernel.org/netdev/20230529113804.GA20300@didi-ThinkCentre-M920t-N000/
    Reviewed-by: Eric Dumazet <edumazet@xxxxxxxxxx>
    Link: https://lore.kernel.org/r/20230531080150.GA20424@didi-ThinkCentre-M920t-N000
    Signed-off-by: Paolo Abeni <pabeni@xxxxxxxxxx>
    Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>

diff --git a/include/net/tcp.h b/include/net/tcp.h
index 0744717f5caa7..5eedd476a38d7 100644
--- a/include/net/tcp.h
+++ b/include/net/tcp.h
@@ -632,6 +632,7 @@ void tcp_reset(struct sock *sk, struct sk_buff *skb);
 void tcp_skb_mark_lost_uncond_verify(struct tcp_sock *tp, struct sk_buff *skb);
 void tcp_fin(struct sock *sk);
 void tcp_check_space(struct sock *sk);
+void tcp_sack_compress_send_ack(struct sock *sk);
 
 /* tcp_timer.c */
 void tcp_init_xmit_timers(struct sock *);
diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index ac44edd6f52e6..5dabb38b857ff 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -4529,7 +4529,7 @@ static void tcp_sack_maybe_coalesce(struct tcp_sock *tp)
 	}
 }
 
-static void tcp_sack_compress_send_ack(struct sock *sk)
+void tcp_sack_compress_send_ack(struct sock *sk)
 {
 	struct tcp_sock *tp = tcp_sk(sk);
 
diff --git a/net/ipv4/tcp_timer.c b/net/ipv4/tcp_timer.c
index cb79127f45c34..0b5d0a2867a8c 100644
--- a/net/ipv4/tcp_timer.c
+++ b/net/ipv4/tcp_timer.c
@@ -290,9 +290,19 @@ static int tcp_write_timeout(struct sock *sk)
 void tcp_delack_timer_handler(struct sock *sk)
 {
 	struct inet_connection_sock *icsk = inet_csk(sk);
+	struct tcp_sock *tp = tcp_sk(sk);
 
-	if (((1 << sk->sk_state) & (TCPF_CLOSE | TCPF_LISTEN)) ||
-	    !(icsk->icsk_ack.pending & ICSK_ACK_TIMER))
+	if ((1 << sk->sk_state) & (TCPF_CLOSE | TCPF_LISTEN))
+		return;
+
+	/* Handling the sack compression case */
+	if (tp->compressed_ack) {
+		tcp_mstamp_refresh(tp);
+		tcp_sack_compress_send_ack(sk);
+		return;
+	}
+
+	if (!(icsk->icsk_ack.pending & ICSK_ACK_TIMER))
 		return;
 
 	if (time_after(icsk->icsk_ack.timeout, jiffies)) {
@@ -312,7 +322,7 @@ void tcp_delack_timer_handler(struct sock *sk)
 			inet_csk_exit_pingpong_mode(sk);
 			icsk->icsk_ack.ato      = TCP_ATO_MIN;
 		}
-		tcp_mstamp_refresh(tcp_sk(sk));
+		tcp_mstamp_refresh(tp);
 		tcp_send_ack(sk);
 		__NET_INC_STATS(sock_net(sk), LINUX_MIB_DELAYEDACKS);
 	}