Index: uipc_mbuf.c
===================================================================
RCS file: /cvs/src/sys/kern/uipc_mbuf.c,v
retrieving revision 1.196
diff -u -p -r1.196 uipc_mbuf.c
--- uipc_mbuf.c	3 Oct 2014 01:11:17 -0000	1.196
+++ uipc_mbuf.c	3 Oct 2014 01:28:55 -0000
@@ -344,14 +344,18 @@ m_free(struct mbuf *m)
 void
 m_extref(struct mbuf *o, struct mbuf *n)
 {
+	int refs = MCLISREFERENCED(o);
+
 	n->m_flags |= o->m_flags & (M_EXT|M_EXTWR);
 
-	mtx_enter(&m_extref_mtx);
+	if (refs)
+		mtx_enter(&m_extref_mtx);
 	n->m_ext.ext_nextref = o->m_ext.ext_nextref;
 	n->m_ext.ext_prevref = o;
 	o->m_ext.ext_nextref = n;
 	n->m_ext.ext_nextref->m_ext.ext_prevref = n;
-	mtx_leave(&m_extref_mtx);
+	if (refs)
+		mtx_leave(&m_extref_mtx);
 
 	MCLREFDEBUGN((n), __FILE__, __LINE__);
 }