[PATCH net-next 3/3] net: tcp: check timeout by icsk->icsk_timeout in tcp_retransmit_timer()

[menglong8.dong@gmail.com]

From: Menglong Dong <imagedong@tencent.com>

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 <imagedong@tencent.com>
---
 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;
 		}
-- 
2.40.1