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, 40 insertions, 0 deletions

diff --git a/net/rxrpc/input.c b/net/rxrpc/input.c
index c89647e..23a5e61 100644
--- a/net/rxrpc/input.c
+++ b/net/rxrpc/input.c
@@ -631,6 +631,43 @@ found:
 }
 
 /*
+ * Process the response to a ping that we sent to find out if we lost an ACK.
+ *
+ * If we got back a ping response 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.
+ */
+static void rxrpc_input_check_for_lost_ack(struct rxrpc_call *call)
+{
+	rxrpc_seq_t top, bottom, seq;
+	bool resend = false;
+
+	spin_lock_bh(&call->lock);
+
+	bottom = call->tx_hard_ack + 1;
+	top = call->acks_lost_top;
+	if (before(bottom, top)) {
+		for (seq = bottom; before_eq(seq, top); seq++) {
+			int ix = seq & RXRPC_RXTX_BUFF_MASK;
+			u8 annotation = call->rxtx_annotations[ix];
+			u8 anno_type = annotation & RXRPC_TX_ANNO_MASK;
+
+			if (anno_type != RXRPC_TX_ANNO_UNACK)
+				continue;
+			annotation &= ~RXRPC_TX_ANNO_MASK;
+			annotation |= RXRPC_TX_ANNO_RETRANS;
+			call->rxtx_annotations[ix] = annotation;
+			resend = true;
+		}
+	}
+
+	spin_unlock_bh(&call->lock);
+
+	if (resend && !test_and_set_bit(RXRPC_CALL_EV_RESEND, &call->events))
+		rxrpc_queue_call(call);
+}
+
+/*
  * Process a ping response.
  */
 static void rxrpc_input_ping_response(struct rxrpc_call *call,
@@ -645,6 +682,9 @@ static void rxrpc_input_ping_response(struct rxrpc_call *call,
 	smp_rmb();
 	ping_serial = call->ping_serial;
 
+	if (orig_serial == call->acks_lost_ping)
+		rxrpc_input_check_for_lost_ack(call);
+
 	if (!test_bit(RXRPC_CALL_PINGING, &call->flags) ||
 	    before(orig_serial, ping_serial))
 		return;