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, 8 insertions, 4 deletions

diff --git a/include/net/request_sock.h b/include/net/request_sock.h
index b01d8dd9ee7c..a51dbd17c2de 100644
--- a/include/net/request_sock.h
+++ b/include/net/request_sock.h
@@ -49,13 +49,16 @@ struct request_sock_ops {
 					   struct request_sock *req);
 };
 
+extern int inet_rtx_syn_ack(struct sock *parent, struct request_sock *req);
+
 /* struct request_sock - mini sock to represent a connection request
  */
 struct request_sock {
 	struct request_sock		*dl_next; /* Must be first member! */
 	u16				mss;
-	u8				retrans;
-	u8				cookie_ts; /* syncookie: encode tcpopts in timestamp */
+	u8				num_retrans; /* number of retransmits */
+	u8				cookie_ts:1; /* syncookie: encode tcpopts in timestamp */
+	u8				num_timeout:7; /* number of timeouts */
 	/* The following two fields can be easily recomputed I think -AK */
 	u32				window_clamp; /* window clamp at creation time */
 	u32				rcv_wnd;	  /* rcv_wnd offered first time */
@@ -231,7 +234,7 @@ static inline int reqsk_queue_removed(struct request_sock_queue *queue,
 {
 	struct listen_sock *lopt = queue->listen_opt;
 
-	if (req->retrans == 0)
+	if (req->num_timeout == 0)
 		--lopt->qlen_young;
 
 	return --lopt->qlen;
@@ -269,7 +272,8 @@ static inline void reqsk_queue_hash_req(struct request_sock_queue *queue,
 	struct listen_sock *lopt = queue->listen_opt;
 
 	req->expires = jiffies + timeout;
-	req->retrans = 0;
+	req->num_retrans = 0;
+	req->num_timeout = 0;
 	req->sk = NULL;
 	req->dl_next = lopt->syn_table[hash];