tun: wake up waitqueues after IFF_UP is set

Currently after setting tap0 link up, the tun code wakes tx/rx waited
queues up in tun_net_open() when .ndo_open() is called, however the
IFF_UP flag has not been set yet. If there's already a wait queue, it
would fail to transmit when checking the IFF_UP flag in tun_sendmsg().
Then the saving vhost_poll_start() will add the wq into wqh until it
is waken up again. Although this works when IFF_UP flag has been set
when tun_chr_poll detects; this is not true if IFF_UP flag has not
been set at that time. Sadly the latter case is a fatal error, as
the wq will never be waken up in future unless later manually
setting link up on purpose.

Fix this by moving the wakeup process into the NETDEV_UP event
notifying process, this makes sure IFF_UP has been set before all
waited queues been waken up.
Signed-off-by: Fei Li <lifei.shirley@bytedance.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/tun.c

@@ -1014,18 +1014,8 @@ static void tun_net_uninit(struct net_device *dev)
 /* Net device open. */
 static int tun_net_open(struct net_device *dev)
 {
-	struct tun_struct *tun = netdev_priv(dev);
-	int i;
-
 	netif_tx_start_all_queues(dev);
 
-	for (i = 0; i < tun->numqueues; i++) {
-		struct tun_file *tfile;
-
-		tfile = rtnl_dereference(tun->tfiles[i]);
-		tfile->socket.sk->sk_write_space(tfile->socket.sk);
-	}
-
 	return 0;
 }
 
@@ -3634,6 +3624,7 @@ static int tun_device_event(struct notifier_block *unused,
 {
 	struct net_device *dev = netdev_notifier_info_to_dev(ptr);
 	struct tun_struct *tun = netdev_priv(dev);
+	int i;
 
 	if (dev->rtnl_link_ops != &tun_link_ops)
 		return NOTIFY_DONE;
@@ -3643,6 +3634,14 @@ static int tun_device_event(struct notifier_block *unused,
 		if (tun_queue_resize(tun))
 			return NOTIFY_BAD;
 		break;
+	case NETDEV_UP:
+		for (i = 0; i < tun->numqueues; i++) {
+			struct tun_file *tfile;
+
+			tfile = rtnl_dereference(tun->tfiles[i]);
+			tfile->socket.sk->sk_write_space(tfile->socket.sk);
+		}
+		break;
 	default:
 		break;
 	}