Index: ifq.c
===================================================================
RCS file: /cvs/src/sys/net/ifq.c,v
retrieving revision 1.35
diff -u -p -r1.35 ifq.c
--- ifq.c	8 Oct 2019 04:18:00 -0000	1.35
+++ ifq.c	25 Jan 2020 06:02:22 -0000
@@ -389,6 +389,45 @@ ifq_dequeue(struct ifqueue *ifq)
 }
 
 int
+ifq_deq_sleep(struct ifqueue *ifq, struct mbuf **mp, int nbio, int priority,
+    const char *wmesg, volatile unsigned int *sleeping,
+    volatile unsigned int *alive)
+{
+	struct mbuf *m;
+	void *cookie;
+	int error;
+
+	ifq_deq_enter(ifq);
+	if (ifq->ifq_len == 0 && nbio)
+		error = EWOULDBLOCK;
+	else {
+		for (;;) {
+			m = ifq->ifq_ops->ifqop_deq_begin(ifq, &cookie);
+			if (m != NULL) {
+				ifq->ifq_ops->ifqop_deq_commit(ifq, m, cookie);
+				ifq->ifq_len--;
+				*mp = m;
+				break;
+			}
+
+			(*sleeping)++;
+			error = msleep_nsec(ifq, &ifq->ifq_mtx,
+			    priority, wmesg, INFSLP);
+			(*sleeping)--;
+			if (error != 0)
+				break;
+			if (!(*alive)) {
+				error = ENXIO;
+				break;
+			}
+		}
+	}
+	ifq_deq_leave(ifq);
+
+	return (error);
+}
+
+int
 ifq_hdatalen(struct ifqueue *ifq)
 {
 	struct mbuf *m;
Index: ifq.h
===================================================================
RCS file: /cvs/src/sys/net/ifq.h,v
retrieving revision 1.26
diff -u -p -r1.26 ifq.h
--- ifq.h	16 Apr 2019 04:04:19 -0000	1.26
+++ ifq.h	25 Jan 2020 06:02:22 -0000
@@ -413,6 +413,11 @@ void		 ifq_q_leave(struct ifqueue *, voi
 void		 ifq_serialize(struct ifqueue *, struct task *);
 void		 ifq_barrier(struct ifqueue *);
 
+
+int		 ifq_deq_sleep(struct ifqueue *, struct mbuf **, int, int,
+		     const char *, volatile unsigned int *,
+		     volatile unsigned int *);
+
 #define	ifq_len(_ifq)			((_ifq)->ifq_len)
 #define	ifq_empty(_ifq)			(ifq_len(_ifq) == 0)
 #define	ifq_set_maxlen(_ifq, _l)	((_ifq)->ifq_maxlen = (_l))
Index: if_tun.c
===================================================================
RCS file: /cvs/src/sys/net/if_tun.c,v
retrieving revision 1.211
diff -u -p -r1.211 if_tun.c
--- if_tun.c	25 Jan 2020 05:28:31 -0000	1.211
+++ if_tun.c	25 Jan 2020 06:02:22 -0000
@@ -766,29 +766,10 @@ tun_dev_read(dev_t dev, struct uio *uio,
 
 	ifp = &sc->sc_if;
 
-	m0 = ifq_dequeue(&ifp->if_snd);
-	if (m0 == NULL) {
-		if (ISSET(ioflag, IO_NDELAY)) {
-			error = EWOULDBLOCK;
-			goto put;
-		}
-
-		do {
-			sc->sc_reading++;
-			error = tsleep_nsec(sc, (PZERO + 1)|PCATCH,
-			    "tunread", INFSLP);
-			sc->sc_reading--;
-			if (error != 0)
-				goto put;
-
-			if (ISSET(sc->sc_flags, TUN_DEAD)) {
-				error = ENXIO;
-				goto put;
-			}
-
-			m0 = ifq_dequeue(&ifp->if_snd);
-		} while (m0 == NULL);
-	}
+	error = ifq_deq_sleep(&ifp->if_snd, &m0, ioflag, (PZERO + 1)|PCATCH,
+	    "tunread", &sc->sc_reading, &sc->sc_dev);
+	if (error != 0)
+		goto put;
 
 #if NBPFILTER > 0
 	if (ifp->if_bpf)