Re: [PATCH net-next v2 3/3] net: tcp: fix unexcepted socket die when snd_wnd is 0

[Menglong Dong]

On Mon, Aug 7, 2023 at 10:20 PM Eric Dumazet <edumazet@xxxxxxxxxx> wrote:
>
> On Mon, Aug 7, 2023 at 3:47 PM <menglong8.dong@xxxxxxxxx> wrote:
> >
> > From: Menglong Dong <imagedong@xxxxxxxxxxx>
> >
> > 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_retransmit_timer()
> > if the 'snd_wnd==0'. 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 timer wheel.
>
> > Sorry that I am not good at timer, but I know the concept of time-wheel.
>
> Can you remove this line, can we keep focused on the actual patch instead ?
>
> Regardless of timer-wheel, a timer can be delayed under load.
>

Okay!

> > The longer of the timer, the rougher it will be. So the timeout is not
> > triggered after TCP_RTO_MAX, but 122877ms as I tested.
> >
> > Therefore, 'tcp_jiffies32 - tp->rcv_tstamp > TCP_RTO_MAX' is always true
> > once the RTO come up to TCP_RTO_MAX, and the socket will die.
> >
> > Fix this by replacing the 'tcp_jiffies32' with '(u32)icsk->icsk_timeout',
> > which is exact the timestamp of the timeout. Meanwhile, using
> > "max(tp->retrans_stamp, tp->rcv_tstamp)" as the last updated timestamp in
> > the receiving path, as "tp->rcv_tstamp" can restart from idle, then
> > tp->rcv_tstamp could already be a long time (minutes or hours) in the
> > past even on the first RTO.
> >
> > Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
> > Link: https://lore.kernel.org/netdev/CADxym3YyMiO+zMD4zj03YPM3FBi-1LHi6gSD2XT8pyAMM096pg@xxxxxxxxxxxxxx/
> > Signed-off-by: Menglong Dong <imagedong@xxxxxxxxxxx>
> > ---
> > v2:
> > - consider the case of the connection restart from idle, as Neal comment
> > ---
> >  net/ipv4/tcp_timer.c | 10 +++++++++-
> >  1 file changed, 9 insertions(+), 1 deletion(-)
> >
> > diff --git a/net/ipv4/tcp_timer.c b/net/ipv4/tcp_timer.c
> > index d45c96c7f5a4..e4b2d8706cae 100644
> > --- a/net/ipv4/tcp_timer.c
> > +++ b/net/ipv4/tcp_timer.c
> > @@ -454,6 +454,14 @@ static void tcp_fastopen_synack_timer(struct sock *sk, struct request_sock *req)
> >                           req->timeout << req->num_timeout, TCP_RTO_MAX);
> >  }
> >
> > +static bool tcp_rtx_probe0_timed_out(struct sock *sk)
> > +{
> > +       struct tcp_sock *tp = tcp_sk(sk);
> > +       u32 last_ts;
> > +
> > +       last_ts = max(tp->retrans_stamp, tp->rcv_tstamp);
>
> u32     retrans_stamp;  /* Timestamp of the last retransmit,
> u32     rcv_tstamp;     /* timestamp of last received ACK (for keepalives) */
>
> Both fields receive tcp_jiffies32 values, which wrap every 2^32 ticks.
>
> So max(A, B) won't work as you expect...
>
> You need to use before(), after() or something like that.

Now I know how the before()/after() works. I thought they
simply compared the arguments.

I'll use brefore/after here instead.

Thanks!
Menglong Dong

>
> https://en.wikipedia.org/wiki/Serial_number_arithmetic#General_Solution
>
>
> > +       return inet_csk(sk)->icsk_timeout - last_ts > TCP_RTO_MAX;
> > +}
> >
> >  /**
> >   *  tcp_retransmit_timer() - The TCP retransmit timeout handler
> > @@ -519,7 +527,7 @@ void tcp_retransmit_timer(struct sock *sk)
> >                                             tp->snd_una, tp->snd_nxt);
> >                 }
> >  #endif
> > -               if (tcp_jiffies32 - tp->rcv_tstamp > TCP_RTO_MAX) {
> > +               if (tcp_rtx_probe0_timed_out(sk)) {
> >                         tcp_write_err(sk);
> >                         goto out;
> >                 }
> > --
> > 2.40.1
> >