From: Yuchung Cheng <ych...@google.com>
Date: Fri, 7 Jun 2019 18:26:33 -0700
> Commit 794200d66273 ("tcp: undo cwnd on Fast Open spurious SYNACK
> retransmit") may cause tcp_fastretrans_alert() to warn about pending
> retransmission in Open state. This is triggered when the Fast Open
> server both sends data and has spurious SYNACK retransmission during
> the handshake, and the data packets were lost or reordered.
>
> The root cause is a bit complicated:
>
> (1) Upon receiving SYN-data: a full socket is created with
> snd_una = ISN + 1 by tcp_create_openreq_child()
>
> (2) On SYNACK timeout the server/sender enters CA_Loss state.
>
> (3) Upon receiving the final ACK to complete the handshake, sender
> does not mark FLAG_SND_UNA_ADVANCED since (1)
>
> Sender then calls tcp_process_loss since state is CA_loss by (2)
>
> (4) tcp_process_loss() does not invoke undo operations but instead
> mark REXMIT_LOST to force retransmission
>
> (5) tcp_rcv_synrecv_state_fastopen() calls tcp_try_undo_loss(). It
> changes state to CA_Open but has positive tp->retrans_out
>
> (6) Next ACK triggers the WARN_ON in tcp_fastretrans_alert()
>
> The step that goes wrong is (4) where the undo operation should
> have been invoked because the ACK successfully acknowledged the
> SYN sequence. This fixes that by specifically checking undo
> when the SYN-ACK sequence is acknowledged. Then after
> tcp_process_loss() the state would be further adjusted based
> in tcp_fastretrans_alert() to avoid triggering the warning in (6).
>
> Fixes: 794200d66273 ("tcp: undo cwnd on Fast Open spurious SYNACK retransmit")
> Signed-off-by: Yuchung Cheng <ych...@google.com>
> Signed-off-by: Neal Cardwell <ncardw...@google.com>
Applied, thank you.
