netdev - Re: [REGRESSION] Warning in tcp_fastretrans_alert() of net/ipv4/tcp

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Date:   Fri, 15 Sep 2017 07:03:26 +0200
From:   Oleksandr Natalenko <oleksandr@...alenko.name>
To:     Neal Cardwell <ncardwell@...gle.com>
Cc:     "David S. Miller" <davem@...emloft.net>,
        Alexey Kuznetsov <kuznet@....inr.ac.ru>,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        Netdev <netdev@...r.kernel.org>,
        Yuchung Cheng <ycheng@...gle.com>
Subject: Re: [REGRESSION] Warning in tcp_fastretrans_alert() of net/ipv4/tcp_input.c

Hi.

I've applied your test patch but it doesn't fix the issue for me since the 
warning is still there.

Were you able to reproduce it?

On pondělí 11. září 2017 1:59:02 CEST Neal Cardwell wrote:
> Thanks for the detailed report!
> 
> I suspect this is due to the following commit, which happened between
> 4.10 and 4.11:
> 
>   89fe18e44f7e tcp: extend F-RTO to catch more spurious timeouts
>  
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?
> id=89fe18e44f7e
> 
> This commit expanded the set of scenarios where we would undo a
> CA_Loss cwnd reduction and return to TCP_CA_Open, but did not include
> a check to see if there were any in-flight retransmissions. I think we
> need a fix like the following:
> 
> diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
> index 659d1baefb2b..730a2de9d2b0 100644
> --- a/net/ipv4/tcp_input.c
> +++ b/net/ipv4/tcp_input.c
> @@ -2439,7 +2439,7 @@ static bool tcp_try_undo_loss(struct sock *sk,
> bool frto_undo)
>  {
>         struct tcp_sock *tp = tcp_sk(sk);
> 
> -       if (frto_undo || tcp_may_undo(tp)) {
> +       if ((frto_undo || tcp_may_undo(tp)) && !tp->retrans_out) {
>                 tcp_undo_cwnd_reduction(sk, true);
> 
>                 DBGUNDO(sk, "partial loss");
> 
> I will try a packetdrill test to see if I can reproduce this issue and
> verify the fix.
> 
> thanks,
> neal