tcp: better retrans tracking for defer-accept

For passive TCP connections using TCP_DEFER_ACCEPT facility,
we incorrectly increment req->retrans each time timeout triggers
while no SYNACK is sent.

SYNACK are not sent for TCP_DEFER_ACCEPT that were established (for
which we received the ACK from client). Only the last SYNACK is sent
so that we can receive again an ACK from client, to move the req into
accept queue. We plan to change this later to avoid the useless
retransmit (and potential problem as this SYNACK could be lost)

TCP_INFO later gives wrong information to user, claiming imaginary
retransmits.

Decouple req->retrans field into two independent fields :

num_retrans : number of retransmit
num_timeout : number of timeouts

num_timeout is the counter that is incremented at each timeout,
regardless of actual SYNACK being sent or not, and used to
compute the exponential timeout.

Introduce inet_rtx_syn_ack() helper to increment num_retrans
only if ->rtx_syn_ack() succeeded.

Use inet_rtx_syn_ack() from tcp_check_req() to increment num_retrans
when we re-send a SYNACK in answer to a (retransmitted) SYN.
Prior to this patch, we were not counting these retransmits.

Change tcp_v[46]_rtx_synack() to increment TCP_MIB_RETRANSSEGS
only if a synack packet was successfully queued.

Reported-by: Yuchung Cheng <ycheng@google.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Julian Anastasov <ja@ssi.bg>
Cc: Vijay Subramanian <subramanian.vijay@gmail.com>
Cc: Elliott Hughes <enh@google.com>
Cc: Neal Cardwell <ncardwell@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 7 insertions, 4 deletions

diff --git a/net/ipv6/tcp_ipv6.c b/net/ipv6/tcp_ipv6.c
index bb6782e84996..c73d0ebde9c8 100644
--- a/net/ipv6/tcp_ipv6.c
+++ b/net/ipv6/tcp_ipv6.c
@@ -495,9 +495,12 @@ static int tcp_v6_rtx_synack(struct sock *sk, struct request_sock *req,
                              struct request_values *rvp)
 {
         struct flowi6 fl6;
+        int res;
 
-        TCP_INC_STATS_BH(sock_net(sk), TCP_MIB_RETRANSSEGS);
-        return tcp_v6_send_synack(sk, NULL, &fl6, req, rvp, 0);
+        res = tcp_v6_send_synack(sk, NULL, &fl6, req, rvp, 0);
+        if (!res)
+                TCP_INC_STATS_BH(sock_net(sk), TCP_MIB_RETRANSSEGS);
+        return res;
 }
 
 static void tcp_v6_reqsk_destructor(struct request_sock *req)
@@ -1364,7 +1367,7 @@ static struct sock * tcp_v6_syn_recv_sock(struct sock *sk, struct sk_buff *skb,
 
         tcp_initialize_rcv_mss(newsk);
         tcp_synack_rtt_meas(newsk, req);
-        newtp->total_retrans = req->retrans;
+        newtp->total_retrans = req->num_retrans;
 
         newinet->inet_daddr = newinet->inet_saddr = LOOPBACK4_IPV6;
         newinet->inet_rcv_saddr = LOOPBACK4_IPV6;
@@ -1866,7 +1869,7 @@ static void get_openreq6(struct seq_file *seq,
                    0,0, /* could print option size, but that is af dependent. */
                    1,   /* timers active (only the expire timer) */
                    jiffies_to_clock_t(ttd),
-                   req->retrans,
+                   req->num_timeout,
                    from_kuid_munged(seq_user_ns(seq), uid),
                    0,  /* non standard timer */
                    0, /* open_requests have no inode */