Re: [PATCH] tcp: fix marked lost packets not being retransmitted

[Neal Cardwell]

On Tue, Jan 14, 2020 at 4:24 AM Pengcheng Yang <yangpc@xxxxxxxxxx> wrote:
>
> 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().
...
> --- a/net/ipv4/tcp_input.c
> +++ b/net/ipv4/tcp_input.c
> @@ -915,9 +915,10 @@ static void tcp_check_sack_reordering(struct sock *sk, const u32 low_seq,
>  /* 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;
>  }

Thanks for finding and fixing this issue, and for providing the very
nice packetdrill test case! The fix looks good to me.

I verified that the packetdrill test fails at the line notated "BUG"
without the patch applied:

fr-retrans-hint-skip-fix.pkt:33: error handling packet: live packet
field tcp_seq: expected: 2001 (0x7d1) vs actual: 4001 (0xfa1)
script packet:  0.137311 . 2001:3001(1000) ack 1
actual packet:  0.137307 . 4001:5001(1000) ack 1 win 256

Also verified that the test passes with the patch applied, and that
our internal SACK and fast recovery tests continue to pass with this
patch applied.

Acked-by: Neal Cardwell <ncardwell@xxxxxxxxxx>
Tested-by: Neal Cardwell <ncardwell@xxxxxxxxxx>

thanks,
neal