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

[Eric Dumazet]

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.

> 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.

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
>