--- src/sys/net/ppp_layer/ppp_tty.c	2006/01/30 14:16:16	1.16
+++ src/sys/net/ppp_layer/ppp_tty.c	2006/02/06 01:49:37	1.17
@@ -409,7 +409,9 @@ pppwrite(struct tty *tp, struct uio *uio
     m0->m_len -= PPP_HDRLEN;
 
     /* call the upper layer to "transmit" it... */
+    lwkt_serialize_enter(sc->sc_if.if_serializer);
     error = pppoutput(&sc->sc_if, m0, &dst, (struct rtentry *)0);
+    lwkt_serialize_exit(sc->sc_if.if_serializer);
     crit_exit();
     return (error);
 }