Re: [PATCH net-next v4 3/4] net: tcp: fix unexcepted socket die when snd_wnd is 0

[Eric Dumazet]

On Fri, Aug 11, 2023 at 5:01 AM <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 can be delayed and be triggered after 122877ms, not
> TCP_RTO_MAX, 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.
>
> However, "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. So we double check the timeout with the duration of the
> retransmission.
>
> Meanwhile, making "2 * TCP_RTO_MAX" as the timeout to avoid the socket
> dying too soon.
>
> Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
> Link: https://lore.kernel.org/netdev/CADxym3YyMiO+zMD4zj03YPM3FBi-1LHi6gSD2XT8pyAMM096pg@xxxxxxxxxxxxxx/
> Signed-off-by: Menglong Dong <imagedong@xxxxxxxxxxx>

Reviewed-by: Eric Dumazet <edumazet@xxxxxxxxxx>