diff --git a/libs/ardour/ardour/audioengine.h b/libs/ardour/ardour/audioengine.h
index b40f1ad27a..3d9f401c80 100644
--- a/libs/ardour/ardour/audioengine.h
+++ b/libs/ardour/ardour/audioengine.h
@@ -135,6 +135,7 @@ class LIBARDOUR_API AudioEngine : public PortManager, public SessionHandlePtr
 	bool running() const { return _running; }
 
 	Glib::Threads::Mutex& process_lock() { return _process_lock; }
+	Glib::Threads::Mutex& latency_lock() { return _latency_lock; }
 	Glib::Threads::RecMutex& state_lock() { return _state_lock; }
 
 	int request_buffer_size (pframes_t samples) {
@@ -268,6 +269,7 @@ class LIBARDOUR_API AudioEngine : public PortManager, public SessionHandlePtr
 	static AudioEngine*       _instance;
 
 	Glib::Threads::Mutex       _process_lock;
+	Glib::Threads::Mutex       _latency_lock;
 	Glib::Threads::RecMutex    _state_lock;
 	Glib::Threads::Cond        session_removed;
 	bool                       session_remove_pending;
diff --git a/libs/ardour/audioengine.cc b/libs/ardour/audioengine.cc
index 9be6248069..6e019ee7f0 100644
--- a/libs/ardour/audioengine.cc
+++ b/libs/ardour/audioengine.cc
@@ -308,6 +308,7 @@ AudioEngine::process_callback (pframes_t nframes)
 			lc = true;
 		}
 		if (lp || lc) {
+			Glib::Threads::Mutex::Lock ll (_latency_lock);
 			tm.release ();
 			/* re-check after releasing lock */
 			if (_session->processing_blocked ()) {
@@ -1479,12 +1480,18 @@ AudioEngine::latency_callback (bool for_playback)
 		return;
 	}
 
-	if (in_process_thread () && ! _session->processing_blocked ()) {
+	if (in_process_thread ()) {
 		/* internal backends emit the latency callback in the rt-callback,
 		 * async to connect/disconnect or port creation/deletion.
 		 * All is fine.
 		 */
-		_session->update_latency (for_playback);
+		Glib::Threads::Mutex::Lock ll (_latency_lock);
+		if (_session->processing_blocked ()) {
+		 /* Except Session::write_one_track() might just have called block_processing() */
+			queue_latency_update (for_playback);
+		} else {
+			_session->update_latency (for_playback);
+		}
 	} else {
 		/* However jack 1/2 emit the callback in sync with creating the port
 		 * (or while handling the connection change).
diff --git a/libs/ardour/session.cc b/libs/ardour/session.cc
index d9bd579e65..7663af6636 100644
--- a/libs/ardour/session.cc
+++ b/libs/ardour/session.cc
@@ -5762,6 +5762,8 @@ Session::write_one_track (Track& track, samplepos_t start, samplepos_t end,
 		// and processing_blocked() is honored before
 		// acquiring thread buffers
 		Glib::Threads::Mutex::Lock lm (_engine.process_lock());
+		/* latency callback may be in process, wait until complete */
+		Glib::Threads::Mutex::Lock lx (_engine.latency_lock());
 	}
 
 	_bounce_processing_active = true;