rxrpc: Make rxrpc_kernel_get_srtt() indicate validity

Fix rxrpc_kernel_get_srtt() to indicate the validity of the returned
smoothed RTT.  If we haven't had any valid samples yet, the SRTT isn't
useful.

Fixes: c410bf0193 ("rxrpc: Fix the excessive initial retransmission timeout")
Signed-off-by: David Howells <dhowells@redhat.com>

fs/afs/fs_probe.c

@@ -161,8 +161,8 @@ responded:
 		}
 	}
 
-	rtt_us = rxrpc_kernel_get_srtt(call->net->socket, call->rxcall);
-	if (rtt_us < server->probe.rtt) {
+	if (rxrpc_kernel_get_srtt(call->net->socket, call->rxcall, &rtt_us) &&
+	    rtt_us < server->probe.rtt) {
 		server->probe.rtt = rtt_us;
 		server->rtt = rtt_us;
 		alist->preferred = index;

fs/afs/vl_probe.c

@@ -92,8 +92,8 @@ responded:
 		}
 	}
 
-	rtt_us = rxrpc_kernel_get_srtt(call->net->socket, call->rxcall);
-	if (rtt_us < server->probe.rtt) {
+	if (rxrpc_kernel_get_srtt(call->net->socket, call->rxcall, &rtt_us) &&
+	    rtt_us < server->probe.rtt) {
 		server->probe.rtt = rtt_us;
 		alist->preferred = index;
 		have_result = true;

include/net/af_rxrpc.h

@@ -59,7 +59,7 @@ bool rxrpc_kernel_abort_call(struct socket *, struct rxrpc_call *,
 void rxrpc_kernel_end_call(struct socket *, struct rxrpc_call *);
 void rxrpc_kernel_get_peer(struct socket *, struct rxrpc_call *,
 			   struct sockaddr_rxrpc *);
-u32 rxrpc_kernel_get_srtt(struct socket *, struct rxrpc_call *);
+bool rxrpc_kernel_get_srtt(struct socket *, struct rxrpc_call *, u32 *);
 int rxrpc_kernel_charge_accept(struct socket *, rxrpc_notify_rx_t,
 			       rxrpc_user_attach_call_t, unsigned long, gfp_t,
 			       unsigned int);

net/rxrpc/peer_object.c

@@ -502,11 +502,21 @@ EXPORT_SYMBOL(rxrpc_kernel_get_peer);
  * rxrpc_kernel_get_srtt - Get a call's peer smoothed RTT
  * @sock: The socket on which the call is in progress.
  * @call: The call to query
+ * @_srtt: Where to store the SRTT value.
  *
- * Get the call's peer smoothed RTT.
+ * Get the call's peer smoothed RTT in uS.
  */
-u32 rxrpc_kernel_get_srtt(struct socket *sock, struct rxrpc_call *call)
+bool rxrpc_kernel_get_srtt(struct socket *sock, struct rxrpc_call *call,
+			   u32 *_srtt)
 {
-	return call->peer->srtt_us >> 3;
+	struct rxrpc_peer *peer = call->peer;
+
+	if (peer->rtt_count == 0) {
+		*_srtt = 1000000; /* 1S */
+		return false;
+	}
+
+	*_srtt = call->peer->srtt_us >> 3;
+	return true;
 }
 EXPORT_SYMBOL(rxrpc_kernel_get_srtt);