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    Subject[PATCH 4.14 49/65] tcp: fix marked lost packets not being retransmitted
    Date
    From: Pengcheng Yang <yangpc@wangsu.com>

    [ Upstream commit e176b1ba476cf36f723cfcc7a9e57f3cb47dec70 ]

    When the packet pointed to by retransmit_skb_hint is unlinked by ACK,
    retransmit_skb_hint will be set to NULL in tcp_clean_rtx_queue().
    If packet loss is detected at this time, retransmit_skb_hint will be set
    to point to the current packet loss in tcp_verify_retransmit_hint(),
    then the packets that were previously marked lost but not retransmitted
    due to the restriction of cwnd will be skipped and cannot be
    retransmitted.

    To fix this, when retransmit_skb_hint is NULL, retransmit_skb_hint can
    be reset only after all marked lost packets are retransmitted
    (retrans_out >= lost_out), otherwise we need to traverse from
    tcp_rtx_queue_head in tcp_xmit_retransmit_queue().

    Packetdrill to demonstrate:

    // Disable RACK and set max_reordering to keep things simple
    0 `sysctl -q net.ipv4.tcp_recovery=0`
    +0 `sysctl -q net.ipv4.tcp_max_reordering=3`

    // Establish a connection
    +0 socket(..., SOCK_STREAM, IPPROTO_TCP) = 3
    +0 setsockopt(3, SOL_SOCKET, SO_REUSEADDR, [1], 4) = 0
    +0 bind(3, ..., ...) = 0
    +0 listen(3, 1) = 0

    +.1 < S 0:0(0) win 32792 <mss 1000,sackOK,nop,nop,nop,wscale 7>
    +0 > S. 0:0(0) ack 1 <...>
    +.01 < . 1:1(0) ack 1 win 257
    +0 accept(3, ..., ...) = 4

    // Send 8 data segments
    +0 write(4, ..., 8000) = 8000
    +0 > P. 1:8001(8000) ack 1

    // Enter recovery and 1:3001 is marked lost
    +.01 < . 1:1(0) ack 1 win 257 <sack 3001:4001,nop,nop>
    +0 < . 1:1(0) ack 1 win 257 <sack 5001:6001 3001:4001,nop,nop>
    +0 < . 1:1(0) ack 1 win 257 <sack 5001:7001 3001:4001,nop,nop>

    // Retransmit 1:1001, now retransmit_skb_hint points to 1001:2001
    +0 > . 1:1001(1000) ack 1

    // 1001:2001 was ACKed causing retransmit_skb_hint to be set to NULL
    +.01 < . 1:1(0) ack 2001 win 257 <sack 5001:8001 3001:4001,nop,nop>
    // Now retransmit_skb_hint points to 4001:5001 which is now marked lost

    // BUG: 2001:3001 was not retransmitted
    +0 > . 2001:3001(1000) ack 1

    Signed-off-by: Pengcheng Yang <yangpc@wangsu.com>
    Acked-by: Neal Cardwell <ncardwell@google.com>
    Tested-by: Neal Cardwell <ncardwell@google.com>
    Signed-off-by: David S. Miller <davem@davemloft.net>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    ---
    net/ipv4/tcp_input.c | 7 ++++---
    1 file changed, 4 insertions(+), 3 deletions(-)

    --- a/net/ipv4/tcp_input.c
    +++ b/net/ipv4/tcp_input.c
    @@ -932,9 +932,10 @@ static void tcp_update_reordering(struct
    /* This must be called before lost_out is incremented */
    static void tcp_verify_retransmit_hint(struct tcp_sock *tp, struct sk_buff *skb)
    {
    - if (!tp->retransmit_skb_hint ||
    - before(TCP_SKB_CB(skb)->seq,
    - TCP_SKB_CB(tp->retransmit_skb_hint)->seq))
    + if ((!tp->retransmit_skb_hint && tp->retrans_out >= tp->lost_out) ||
    + (tp->retransmit_skb_hint &&
    + before(TCP_SKB_CB(skb)->seq,
    + TCP_SKB_CB(tp->retransmit_skb_hint)->seq)))
    tp->retransmit_skb_hint = skb;
    }


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