diff --git a/sys/dev/usb/serial/usb_serial.c b/sys/dev/usb/serial/usb_serial.c
--- a/sys/dev/usb/serial/usb_serial.c
+++ b/sys/dev/usb/serial/usb_serial.c
@@ -153,7 +153,7 @@
 static void	ucom_detach_tty(struct ucom_super_softc *, struct ucom_softc *);
 static int	ucom_queue_command(struct ucom_softc *,
 		    usb_proc_callback_t *, struct termios *pt,
-		    struct usb_proc_msg *t0, struct usb_proc_msg *t1);
+		    struct usb_proc_msg *t0, struct usb_proc_msg *t1, bool wait);
 static void	ucom_shutdown(struct ucom_softc *);
 static void	ucom_ring(struct ucom_softc *, uint8_t);
 static void	ucom_break(struct ucom_softc *, uint8_t);
@@ -592,10 +592,43 @@
 	}
 }
 
+static void
+ucom_command_barrier_cb(struct usb_proc_msg *msg __unused)
+{
+	/* NOP */
+}
+
+/*
+ * ucom_command_barrier inserts a dummy task and waits for it so that we can be
+ * certain that previously enqueued tasks are finished before returning back to
+ * the tty layer.
+ */
+static int
+ucom_command_barrier(struct ucom_softc *sc)
+{
+	struct ucom_super_softc *ssc = sc->sc_super;
+	struct usb_proc_msg dummy = { .pm_callback = ucom_command_barrier_cb };
+	struct usb_proc_msg *task;
+	int error;
+
+	UCOM_MTX_ASSERT(sc, MA_OWNED);
+
+	if (usb_proc_is_gone(&ssc->sc_tq)) {
+		DPRINTF("proc is gone\n");
+		return (ENXIO);         /* nothing to do */
+	}
+
+	task = usb_proc_msignal(&ssc->sc_tq, &dummy, &dummy);
+	error = usb_proc_mwait_sig(&ssc->sc_tq, task, task);
+	if (error == 0 && sc->sc_tty != NULL && tty_gone(sc->sc_tty))
+		error = ENXIO;
+	return (error);
+}
+
 static int
 ucom_queue_command(struct ucom_softc *sc,
     usb_proc_callback_t *fn, struct termios *pt,
-    struct usb_proc_msg *t0, struct usb_proc_msg *t1)
+    struct usb_proc_msg *t0, struct usb_proc_msg *t1, bool wait)
 {
 	struct ucom_super_softc *ssc = sc->sc_super;
 	struct ucom_param_task *task;
@@ -629,7 +662,7 @@
 	/*
 	 * Closing or opening the device should be synchronous.
 	 */
-	if (fn == ucom_cfg_close || fn == ucom_cfg_open) {
+	if (wait) {
 		error = usb_proc_mwait_sig(&ssc->sc_tq, t0, t1);
 
 		/* usb_proc_mwait_sig may have dropped the tty lock. */
@@ -793,14 +826,17 @@
 
 	error = ucom_queue_command(sc, ucom_cfg_open, NULL,
 	    &sc->sc_open_task[0].hdr,
-	    &sc->sc_open_task[1].hdr);
+	    &sc->sc_open_task[1].hdr, true);
 	if (error != 0)
 		goto out;
 
-	/* Queue transfer enable command last */
+	/*
+	 * Queue transfer enable command last, we'll have a barrier later so we
+	 * don't need to wait on this to complete specifically.
+	 */
 	error = ucom_queue_command(sc, ucom_cfg_start_transfers, NULL,
 	    &sc->sc_start_task[0].hdr,
-	    &sc->sc_start_task[1].hdr);
+	    &sc->sc_start_task[1].hdr, true);
 	if (error != 0)
 		goto out;
 
@@ -813,6 +849,7 @@
 
 	ucom_status_change(sc);
 
+	error = ucom_command_barrier(sc);
 out:
 	return (error);
 }
@@ -852,7 +889,7 @@
 
 	(void)ucom_queue_command(sc, ucom_cfg_close, NULL,
 	    &sc->sc_close_task[0].hdr,
-	    &sc->sc_close_task[1].hdr);
+	    &sc->sc_close_task[1].hdr, true);
 
 	sc->sc_flag &= ~(UCOM_FLAG_HL_READY | UCOM_FLAG_RTS_IFLOW);
 
@@ -933,11 +970,15 @@
 #endif
 	case TIOCSBRK:
 		ucom_break(sc, 1);
-		error = 0;
+		error = ucom_command_barrier(sc);
+		if (error == ENXIO)
+			error = ENODEV;
 		break;
 	case TIOCCBRK:
 		ucom_break(sc, 0);
-		error = 0;
+		error = ucom_command_barrier(sc);
+		if (error == ENXIO)
+			error = ENODEV;
 		break;
 	default:
 		if (sc->sc_callback->ucom_ioctl) {
@@ -1097,7 +1138,7 @@
 	 */
 	(void)ucom_queue_command(sc, ucom_cfg_line_state, NULL,
 	    &sc->sc_line_state_task[0].hdr,
-	    &sc->sc_line_state_task[1].hdr);
+	    &sc->sc_line_state_task[1].hdr, false);
 }
 
 static void
@@ -1255,7 +1296,7 @@
 
 	(void)ucom_queue_command(sc, ucom_cfg_status_change, NULL,
 	    &sc->sc_status_task[0].hdr,
-	    &sc->sc_status_task[1].hdr);
+	    &sc->sc_status_task[1].hdr, true);
 }
 
 static void
@@ -1329,14 +1370,14 @@
 	/* Queue baud rate programming command first */
 	error = ucom_queue_command(sc, ucom_cfg_param, t,
 	    &sc->sc_param_task[0].hdr,
-	    &sc->sc_param_task[1].hdr);
+	    &sc->sc_param_task[1].hdr, true);
 	if (error != 0)
 		goto done;
 
 	/* Queue transfer enable command last */
 	error = ucom_queue_command(sc, ucom_cfg_start_transfers, NULL,
 	    &sc->sc_start_task[0].hdr,
-	    &sc->sc_start_task[1].hdr);
+	    &sc->sc_start_task[1].hdr, true);
 	if (error != 0)
 		goto done;
 
@@ -1346,6 +1387,8 @@
 		sc->sc_flag &= ~UCOM_FLAG_RTS_IFLOW;
 		ucom_modem(tp, SER_RTS, 0);
 	}
+
+	error = ucom_command_barrier(sc);
 done:
 	if (error) {
 		if (opened) {