On Fri, Jul 28, 2023 at 7:25 AM Neal Cardwell [off-list ref] wrote:

On Fri, Jul 28, 2023 at 1:50 AM Eric Dumazet [off-list ref] wrote:

quoted

On Fri, Jul 28, 2023 at 8:25 AM Menglong Dong [off-list ref] wrote:

quoted

On Fri, Jul 28, 2023 at 12:44 PM Neal Cardwell [off-list ref] wrote:

quoted

On Thu, Jul 27, 2023 at 7:57 PM Menglong Dong [off-list ref] wrote:

quoted

On Fri, Jul 28, 2023 at 3:31 AM Eric Dumazet [off-list ref] wrote:

quoted

On Thu, Jul 27, 2023 at 2:52 PM [off-list ref] wrote:

quoted

From: Menglong Dong <redacted>

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.
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.

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 <redacted>
---
 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_RTO_MAX) {
+               /* It's a little rough here, we regard any valid packet that
+                * update tp->rcv_tstamp as the reply of the retransmitted
+                * packet.
+                */
+               if ((u32)icsk->icsk_timeout - tp->rcv_tstamp > 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.

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.

neal