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From:   Menglong Dong <menglong8.dong@gmail.com>
Date:   Mon, 31 Jul 2023 11:41:38 +0800
Message-ID: <CADxym3YgR36hoTEHUbCYUzYkiXpjnSuVEZWnvoqbMo8MoTCtnQ@mail.gmail.com>
Subject: Re: [PATCH net-next 3/3] net: tcp: check timeout by
 icsk->icsk_timeout in tcp_retransmit_timer()
To:     Neal Cardwell <ncardwell@google.com>
Cc:     Eric Dumazet <edumazet@google.com>, davem@davemloft.net,
        kuba@kernel.org, pabeni@redhat.com, dsahern@kernel.org,
        netdev@vger.kernel.org, linux-kernel@vger.kernel.org,
        Menglong Dong <imagedong@tencent.com>,
        Yuchung Cheng <ycheng@google.com>,
        Soheil Hassas Yeganeh <soheil@google.com>
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On Sat, Jul 29, 2023 at 1:15=E2=80=AFAM Neal Cardwell <ncardwell@google.com=
> wrote:
>
> On Fri, Jul 28, 2023 at 7:25=E2=80=AFAM Neal Cardwell <ncardwell@google.c=
om> wrote:
> >
> > On Fri, Jul 28, 2023 at 1:50=E2=80=AFAM Eric Dumazet <edumazet@google.c=
om> wrote:
> > >
> > > On Fri, Jul 28, 2023 at 8:25=E2=80=AFAM Menglong Dong <menglong8.dong=
@gmail.com> wrote:
> > > >
> > > > On Fri, Jul 28, 2023 at 12:44=E2=80=AFPM Neal Cardwell <ncardwell@g=
oogle.com> wrote:
> > > > >
> > > > > On Thu, Jul 27, 2023 at 7:57=E2=80=AFPM Menglong Dong <menglong8.=
dong@gmail.com> wrote:
> > > > > >
> > > > > > On Fri, Jul 28, 2023 at 3:31=E2=80=AFAM Eric Dumazet <edumazet@=
google.com> wrote:
> > > > > > >
> > > > > > > On Thu, Jul 27, 2023 at 2:52=E2=80=AFPM <menglong8.dong@gmail=
.com> wrote:
> > > > > > > >
> > > > > > > > From: Menglong Dong <imagedong@tencent.com>
> > > > > > > >
> > > > > > > > In tcp_retransmit_timer(), a window shrunk connection will =
be regarded
> > > > > > > > as timeout if 'tcp_jiffies32 - tp->rcv_tstamp > TCP_RTO_MAX=
'. This is not
> > > > > > > > right all the time.
> > > > > > > >
> > > > > > > > The retransmits will become zero-window probes in tcp_retra=
nsmit_timer()
> > > > > > > > if the 'snd_wnd=3D=3D0'. Therefore, the icsk->icsk_rto will=
 come up to
> > > > > > > > TCP_RTO_MAX sooner or later.
> > > > > > > >
> > > > > > > > However, the timer is not precise enough, as it base on tim=
er wheel.
> > > > > > > > Sorry that I am not good at timer, but I know the concept o=
f time-wheel.
> > > > > > > > The longer of the timer, the rougher it will be. So the tim=
eout is not
> > > > > > > > triggered after TCP_RTO_MAX, but 122877ms as I tested.
> > > > > > > >
> > > > > > > > Therefore, 'tcp_jiffies32 - tp->rcv_tstamp > TCP_RTO_MAX' i=
s always true
> > > > > > > > once the RTO come up to TCP_RTO_MAX.
> > > > > > > >
> > > > > > > > Fix this by replacing the 'tcp_jiffies32' with '(u32)icsk->=
icsk_timeout',
> > > > > > > > which is exact the timestamp of the timeout.
> > > > > > > >
> > > > > > > > Signed-off-by: Menglong Dong <imagedong@tencent.com>
> > > > > > > > ---
> > > > > > > >  net/ipv4/tcp_timer.c | 6 +++++-
> > > > > > > >  1 file changed, 5 insertions(+), 1 deletion(-)
> > > > > > > >
> > > > > > > > diff --git a/net/ipv4/tcp_timer.c b/net/ipv4/tcp_timer.c
> > > > > > > > index 470f581eedd4..3a20db15a186 100644
> > > > > > > > --- a/net/ipv4/tcp_timer.c
> > > > > > > > +++ b/net/ipv4/tcp_timer.c
> > > > > > > > @@ -511,7 +511,11 @@ void tcp_retransmit_timer(struct sock =
*sk)
> > > > > > > >                                             tp->snd_una, tp=
->snd_nxt);
> > > > > > > >                 }
> > > > > > > >  #endif
> > > > > > > > -               if (tcp_jiffies32 - tp->rcv_tstamp > TCP_RT=
O_MAX) {
> > > > > > > > +               /* It's a little rough here, we regard any =
valid packet that
> > > > > > > > +                * update tp->rcv_tstamp as the reply of th=
e retransmitted
> > > > > > > > +                * packet.
> > > > > > > > +                */
> > > > > > > > +               if ((u32)icsk->icsk_timeout - tp->rcv_tstam=
p > TCP_RTO_MAX) {
> > > > > > > >                         tcp_write_err(sk);
> > > > > > > >                         goto out;
> > > > > > > >                 }
>
> One potential pre-existing issue with this logic: if the connection is
> restarting from idle, then tp->rcv_tstamp could already be a long time
> (minutes or hours) in the past even on the first RTO, in which case
> the very first RTO that found a zero tp->snd_wnd  would find this
> check returns true, and would destroy the connection immediately. This
> seems extremely brittle.

Yes, this scenario can happen and cause the connection
break unexpectedly.

>
> AFAICT it would be safer to replace this logic with a call to the
> standard tcp_write_timeout() logic that has a more robust check to see
> if the connection should be destroyed.

Yes, we need a more robust check here. But I think tcp_write_timeout()
is not a good choice. The icsk->icsk_retransmits won't increase and
can keep being 0 in this scenario, which makes tcp_write_timeout()
always return 0.

Enn...let me think again.

>
> neal