rxrpc: Add a timeout for detecting lost ACKs/lost DATA

Add an extra timeout that is set/updated when we send a DATA packet that
has the request-ack flag set.  This allows us to detect if we don't get an
ACK in response to the latest flagged packet.

The ACK packet is adjudged to have been lost if it doesn't turn up within
2*RTT of the transmission.

If the timeout occurs, we schedule the sending of a PING ACK to find out
the state of the other side.  If a new DATA packet is ready to go sooner,
we cancel the sending of the ping and set the request-ack flag on that
instead.

If we get back a PING-RESPONSE ACK that indicates a lower tx_top than what
we had at the time of the ping transmission, we adjudge all the DATA
packets sent between the response tx_top and the ping-time tx_top to have
been lost and retransmit immediately.

Rather than sending a PING ACK, we could just pick a DATA packet and
speculatively retransmit that with request-ack set.  It should result in
either a REQUESTED ACK or a DUPLICATE ACK which we can then use in lieu the
a PING-RESPONSE ACK mentioned above.

Signed-off-by: David Howells <dhowells@redhat.com>

1 files changed, 18 insertions, 2 deletions

diff --git a/net/rxrpc/output.c b/net/rxrpc/output.c
index f47659c7b224..efe06edce189 100644
--- a/net/rxrpc/output.c
+++ b/net/rxrpc/output.c
@@ -95,7 +95,8 @@ static size_t rxrpc_fill_out_ack(struct rxrpc_connection *conn,
 /*
  * Send an ACK call packet.
  */
-int rxrpc_send_ack_packet(struct rxrpc_call *call, bool ping)
+int rxrpc_send_ack_packet(struct rxrpc_call *call, bool ping,
+			  rxrpc_serial_t *_serial)
 {
 	struct rxrpc_connection *conn = NULL;
 	struct rxrpc_ack_buffer *pkt;
@@ -165,6 +166,8 @@ int rxrpc_send_ack_packet(struct rxrpc_call *call, bool ping)
 			   ntohl(pkt->ack.firstPacket),
 			   ntohl(pkt->ack.serial),
 			   pkt->ack.reason, pkt->ack.nAcks);
+	if (_serial)
+		*_serial = serial;
 
 	if (ping) {
 		call->ping_serial = serial;
@@ -323,7 +326,8 @@ int rxrpc_send_data_packet(struct rxrpc_call *call, struct sk_buff *skb,
 	 * ACKs if a DATA packet appears to have been lost.
 	 */
 	if (!(sp->hdr.flags & RXRPC_LAST_PACKET) &&
-	    (retrans ||
+	    (test_and_clear_bit(RXRPC_CALL_EV_ACK_LOST, &call->events) ||
+	     retrans ||
 	     call->cong_mode == RXRPC_CALL_SLOW_START ||
 	     (call->peer->rtt_usage < 3 && sp->hdr.seq & 1) ||
 	     ktime_before(ktime_add_ms(call->peer->rtt_last_req, 1000),
@@ -370,6 +374,18 @@ done:
 		if (whdr.flags & RXRPC_REQUEST_ACK) {
 			call->peer->rtt_last_req = now;
 			trace_rxrpc_rtt_tx(call, rxrpc_rtt_tx_data, serial);
+			if (call->peer->rtt_usage > 1) {
+				unsigned long nowj = jiffies, ack_lost_at;
+
+				ack_lost_at = nsecs_to_jiffies(2 * call->peer->rtt);
+				if (ack_lost_at < 1)
+					ack_lost_at = 1;
+
+				ack_lost_at += nowj;
+				WRITE_ONCE(call->ack_lost_at, ack_lost_at);
+				rxrpc_reduce_call_timer(call, ack_lost_at, nowj,
+							rxrpc_timer_set_for_lost_ack);
+			}
 		}
 	}
 	_leave(" = %d [%u]", ret, call->peer->maxdata);