diff --git a/libs/ardour/audioengine.cc b/libs/ardour/audioengine.cc
index 1102b9a812..b0c8ae6d6d 100644
--- a/libs/ardour/audioengine.cc
+++ b/libs/ardour/audioengine.cc
@@ -288,13 +288,27 @@ AudioEngine::process_callback (pframes_t nframes)
 	 * port registration (usually while ardour holds the process-lock
 	 * or with _adding_routes_in_progress or _route_deletion_in_progress set,
 	 * potentially while processing in parallel.
+	 *
+	 * Note: this must be done without holding the _process_lock
 	 */
 	if (_session) {
+		bool lp = false;
+		bool lc = false;
 		if (g_atomic_int_compare_and_exchange (&_pending_playback_latency_callback, 1, 0)) {
-			_session->update_latency (true);
+			lp = true;
 		}
 		if (g_atomic_int_compare_and_exchange (&_pending_capture_latency_callback, 1, 0)) {
-			_session->update_latency (false);
+			lc = true;
+		}
+		if (lp || lc) {
+			tm.release ();
+			if (lp) {
+				_session->update_latency (true);
+			}
+			if (lc) {
+				_session->update_latency (false);
+			}
+			tm.acquire ();
 		}
 	}
 
diff --git a/libs/ardour/route.cc b/libs/ardour/route.cc
index 52a194ad7c..f925543558 100644
--- a/libs/ardour/route.cc
+++ b/libs/ardour/route.cc
@@ -4861,6 +4861,8 @@ Route::update_port_latencies (PortSet& from, PortSet& to, bool playback, samplec
 		p->set_private_latency_range (all_connections, playback);
 	}
 
+	DEBUG_TRACE (DEBUG::LatencyRoute, string_compose ("%1: priv. port L(%2) = (%3, %4) + %5\n", _name, playback ? "playback" : "capture", all_connections.min, all_connections.max, our_latency));
+
 	/* set the ports "in the direction of the flow" to the same value as above plus our own signal latency */
 
 	all_connections.min += our_latency;