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From:   Eric Dumazet <edumazet@google.com>
Date:   Mon, 3 Feb 2020 07:54:42 -0800
Message-ID: <CANn89iLNRKGhkqDbx+Q38sB=rV8q=u8-ZsbmpHzcnJSHXdu9Lg@mail.gmail.com>
Subject: Re: [PATCH 2/3] tcp: Reduce SYN resend delay if a suspicous ACK is received
To:     David Laight <David.Laight@aculab.com>
Cc:     Eric Dumazet <eric.dumazet@gmail.com>,
        Neal Cardwell <ncardwell@google.com>,
        "sjpark@amazon.com" <sjpark@amazon.com>,
        David Miller <davem@davemloft.net>,
        "shuah@kernel.org" <shuah@kernel.org>,
        Netdev <netdev@vger.kernel.org>,
        "linux-kselftest@vger.kernel.org" <linux-kselftest@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        "sj38.park@gmail.com" <sj38.park@gmail.com>,
        "aams@amazon.com" <aams@amazon.com>,
        SeongJae Park <sjpark@amazon.de>,
        Yuchung Cheng <ycheng@google.com>
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On Mon, Feb 3, 2020 at 7:40 AM David Laight <David.Laight@aculab.com> wrote:
>
> 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.

Please take a look at packetdrill, there is no possibility for it to
send more than one packet at a time.

Even if we modify packetdrill adding the possibility of feeding
packets to its tun device from multiple threads,
the race is tiny and you would have to run the packetdrill thousands
of times to eventually trigger the race once.

While the test SeongJae provided is using two threads and regular TCP
stack over loopback interface,
it triggers the race more reliably.