Index: sys/dev/usb/ucom.c
===================================================================
RCS file: /cvsroot/src/sys/dev/usb/ucom.c,v
retrieving revision 1.117
diff -u -p -r1.117 ucom.c
--- sys/dev/usb/ucom.c	4 Dec 2016 10:12:35 -0000	1.117
+++ sys/dev/usb/ucom.c	14 Dec 2016 09:48:12 -0000
@@ -1608,6 +1608,7 @@ ucomreadcb(struct usbd_xfer *xfer, void 
 		return;
 	}
 
+	mutex_exit(&sc->sc_lock);
 	if (sc->sc_methods->ucom_read != NULL) {
 		sc->sc_methods->ucom_read(sc->sc_parent, sc->sc_portno,
 		    &cp, &cc);
@@ -1617,6 +1618,15 @@ ucomreadcb(struct usbd_xfer *xfer, void 
 
 	ub->ub_len = cc;
 
+	mutex_enter(&sc->sc_lock);
+	if (sc->sc_dying) {
+		if (--sc->sc_refcnt < 0)
+			cv_broadcast(&sc->sc_detachcv);
+		mutex_exit(&sc->sc_lock);
+		DPRINTF("... dying", 0, 0, 0, 0);
+		return;
+	}
+
 	SIMPLEQ_INSERT_TAIL(&sc->sc_ibuff_full, ub, ub_link);
 
 	ucom_read_complete(sc);