netdev - Re: [PATCH net 4/5] tcp: prevent bogus undos when SACK is not enabled

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Date:   Wed, 7 Mar 2018 15:19:22 -0500
From:   Neal Cardwell <ncardwell@...gle.com>
To:     Ilpo Järvinen <ilpo.jarvinen@...sinki.fi>
Cc:     Netdev <netdev@...r.kernel.org>, Yuchung Cheng <ycheng@...gle.com>,
        Eric Dumazet <edumazet@...gle.com>
Subject: Re: [PATCH net 4/5] tcp: prevent bogus undos when SACK is not enabled

On Wed, Mar 7, 2018 at 7:59 AM, Ilpo Järvinen <ilpo.jarvinen@...sinki.fi> wrote:
> A bogus undo may/will trigger when the loss recovery state is
> kept until snd_una is above high_seq. If tcp_any_retrans_done
> is zero, retrans_stamp is cleared in this transient state. On
> the next ACK, tcp_try_undo_recovery again executes and
> tcp_may_undo will always return true because tcp_packet_delayed
> has this condition:
>     return !tp->retrans_stamp || ...
>
> Check for the false fast retransmit transient condition in
> tcp_packet_delayed to avoid bogus undos. Since snd_una may have
> advanced on this ACK but CA state still remains unchanged,
> prior_snd_una needs to be passed instead of tp->snd_una.

This one also seems like a case where it would be nice to have a
specific packet-by-packet example, or trace, or packetdrill scenario.
Something that we might be able to translate into a test, or at least
to document the issue more explicitly.

Thanks!
neal