netdev - RE: [PATCH 2/3] tcp: Reduce SYN resend delay if a suspicous ACK is received

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Date:   Mon, 3 Feb 2020 15:40:04 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Eric Dumazet' <eric.dumazet@...il.com>,
        Neal Cardwell <ncardwell@...gle.com>
CC:     "sjpark@...zon.com" <sjpark@...zon.com>,
        Eric Dumazet <edumazet@...gle.com>,
        David Miller <davem@...emloft.net>,
        "shuah@...nel.org" <shuah@...nel.org>,
        Netdev <netdev@...r.kernel.org>,
        "linux-kselftest@...r.kernel.org" <linux-kselftest@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        "sj38.park@...il.com" <sj38.park@...il.com>,
        "aams@...zon.com" <aams@...zon.com>,
        SeongJae Park <sjpark@...zon.de>,
        Yuchung Cheng <ycheng@...gle.com>
Subject: RE: [PATCH 2/3] tcp: Reduce SYN resend delay if a suspicous ACK is
 received

From: Eric Dumazet
> Sent: 31 January 2020 22:54
> On 1/31/20 2:11 PM, Neal Cardwell wrote:
> 
> > I looked into fixing this, but my quick reading of the Linux
> > tcp_rcv_state_process() code is that it should behave correctly and
> > that a connection in FIN_WAIT_1 that receives a FIN/ACK should move to
> > TIME_WAIT.
> >
> > SeongJae, do you happen to have a tcpdump trace of the problematic
> > sequence where the "process A" ends up in FIN_WAIT_2 when it should be
> > in TIME_WAIT?
> >
> > If I have time I will try to construct a packetdrill case to verify
> > the behavior in this case.
> 
> Unfortunately you wont be able to reproduce the issue with packetdrill,
> since it involved packets being processed at the same time (race window)

You might be able to force the timing race by adding a sleep
in one of the code paths.

No good for a regression test, but ok for code testing.

	David

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