diff --git a/libs/ardour/ardour/session.h b/libs/ardour/ardour/session.h
index acddc02539..9326bedb35 100644
--- a/libs/ardour/ardour/session.h
+++ b/libs/ardour/ardour/session.h
@@ -304,7 +304,7 @@ public:
 
 	void refill_all_track_buffers ();
 	Butler* butler() { return _butler; }
-	void butler_transport_work ();
+	void butler_transport_work (bool have_process_lock = false);
 
 	void refresh_disk_space ();
 
diff --git a/libs/ardour/session_export.cc b/libs/ardour/session_export.cc
index 78aec6c759..f167b17f66 100644
--- a/libs/ardour/session_export.cc
+++ b/libs/ardour/session_export.cc
@@ -300,7 +300,7 @@ Session::process_export_fw (pframes_t nframes)
 		}
 
 		set_transport_speed (1.0, false, false, false);
-		butler_transport_work ();
+		butler_transport_work (true);
 		g_atomic_int_set (&_butler->should_do_transport_work, 0);
 		butler_completed_transport_work ();
 		/* Session::process_with_events () sets _remaining_latency_preroll = 0
diff --git a/libs/ardour/session_transport.cc b/libs/ardour/session_transport.cc
index 45275f6980..803012c3c1 100644
--- a/libs/ardour/session_transport.cc
+++ b/libs/ardour/session_transport.cc
@@ -1099,7 +1099,7 @@ Session::solo_selection (StripableList &list, bool new_state)
 
 
 void
-Session::butler_transport_work ()
+Session::butler_transport_work (bool have_process_lock)
 {
 	/* Note: this function executes in the butler thread context */
 
@@ -1154,7 +1154,10 @@ Session::butler_transport_work ()
 	if (ptw & PostTransportAdjustPlaybackBuffering) {
 		/* need to prevent concurrency with ARDOUR::Reader::run(),
 		 * DiskWriter::adjust_buffering() re-allocates the ringbuffer */
-		Glib::Threads::Mutex::Lock lx (AudioEngine::instance()->process_lock ());
+		Glib::Threads::Mutex::Lock lx (AudioEngine::instance()->process_lock (), Glib::Threads::NOT_LOCK);
+		if (!have_process_lock) {
+			lx.acquire ();
+		}
 		for (RouteList::iterator i = r->begin(); i != r->end(); ++i) {
 			boost::shared_ptr<Track> tr = boost::dynamic_pointer_cast<Track> (*i);
 			if (tr) {
@@ -1172,7 +1175,10 @@ Session::butler_transport_work ()
 	if (ptw & PostTransportAdjustCaptureBuffering) {
 		/* need to prevent concurrency with ARDOUR::DiskWriter::run(),
 		 * DiskWriter::adjust_buffering() re-allocates the ringbuffer */
-		Glib::Threads::Mutex::Lock lx (AudioEngine::instance()->process_lock ());
+		Glib::Threads::Mutex::Lock lx (AudioEngine::instance()->process_lock (), Glib::Threads::NOT_LOCK);
+		if (!have_process_lock) {
+			lx.acquire ();
+		}
 		for (RouteList::iterator i = r->begin(); i != r->end(); ++i) {
 			boost::shared_ptr<Track> tr = boost::dynamic_pointer_cast<Track> (*i);
 			if (tr) {