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Message-ID: <CADVnQym6OPVRcJ6PdR3hjN5Krcn0pugshdLZsrnzNQe1c52HXA@mail.gmail.com>
Date: Thu, 15 Oct 2020 16:22:21 -0400
From: Neal Cardwell <ncardwell@...gle.com>
To: Apollon Oikonomopoulos <apoikos@...sg.gr>
Cc: Netdev <netdev@...r.kernel.org>,
Eric Dumazet <edumazet@...gle.com>,
Yuchung Cheng <ycheng@...gle.com>,
Soheil Hassas Yeganeh <soheil@...gle.com>
Subject: Re: TCP sender stuck in persist despite peer advertising non-zero window
On Thu, Oct 15, 2020 at 2:31 PM Apollon Oikonomopoulos <apoikos@...sg.gr> wrote:
>
> Hi,
>
> I'm trying to debug a (possible) TCP issue we have been encountering
> sporadically during the past couple of years. Currently we're running
> 4.9.144, but we've been observing this since at least 3.16.
>
> Tl;DR: I believe we are seeing a case where snd_wl1 fails to be properly
> updated, leading to inability to recover from a TCP persist state and
> would appreciate some help debugging this.
Thanks for the detailed report and diagnosis. I think we may need a
fix something like the following patch below.
Eric/Yuchung/Soheil, what do you think?
commit 42b37c72aa73aaabd0c01b8c05c2205236279021
Author: Neal Cardwell <ncardwell@...gle.com>
Date: Thu Oct 15 16:06:11 2020 -0400
tcp: fix to update snd_wl1 in bulk receiver fast path
In the header prediction fast path for a bulk data receiver, if no
data is newly acknowledged then we do not call tcp_ack() and do not
call tcp_ack_update_window(). This means that a bulk receiver that
receives large amounts of data can have the incoming sequence numbers
wrap, so that the check in tcp_may_update_window fails:
after(ack_seq, tp->snd_wl1)
The fix is to update snd_wl1 in the header prediction fast path for a
bulk data receiver, so that it keeps up and does not see wrapping
problems.
Signed-off-by: Neal Cardwell <ncardwell@...gle.com>
Reported-By: Apollon Oikonomopoulos <apoikos@...sg.gr>
diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index b1ce2054291d..75be97f6a7da 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -5766,6 +5766,8 @@ void tcp_rcv_established(struct sock *sk, struct
sk_buff *skb)
tcp_data_snd_check(sk);
if (!inet_csk_ack_scheduled(sk))
goto no_ack;
+ } else {
+ tcp_update_wl(tp, TCP_SKB_CB(skb)->seq);
}
__tcp_ack_snd_check(sk, 0);
neal
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