Re: How to find an unused TAP device with kernel 3.8 ?
From: Jason Wang
Date: Mon Jan 28 2013 - 04:38:27 EST
On Saturday, January 26, 2013 11:29:49 AM Toralf Förster wrote:
> With kernel 3.8. I just realized that all pre-defined TAP devices are
> RUNNING even if no virtual linux system is using it. Now I'm wondering
> whether this is a new feature of the upcoming kernel, a bug of the old -
> and how I can avoid that.
>
> Background : When I start a user mode linux instance, I currently grep
> for the next free tap device in this way :
>
> Code:
> for t in $(ifconfig | grep "^tap" | cut -f1 -d:)
> do
> ethtool $t | grep -q 'Link detected: no'
> if [[ $? -eq 0 ]]; then
> NET="tuntap,$t"
> break
> fi
> done
>
> but this doesn't work now anymore.
>
> Here is the diff in dmesg
>
> $ diff 3.7.1 3.8.0-rc1+ | grep UP
>
> < br0: flags=4355<UP,BROADCAST,PROMISC,MULTICAST> mtu 1500
>
> > br0: flags=4419<UP,BROADCAST,RUNNING,PROMISC,MULTICAST> mtu 1500
>
> < tap0: flags=4099<UP,BROADCAST,MULTICAST> mtu 1500
>
> > tap0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST> mtu 1500
>
> more details : http://forums.gentoo.org/viewtopic-p-7211498.html#7211498
Looks like something wrong with the carrier detection of tap.
Please try the following patch to see if it helps.
Thanks.
diff --git a/drivers/net/tun.c b/drivers/net/tun.c
index cc09b67..d8a53c3 100644
--- a/drivers/net/tun.c
+++ b/drivers/net/tun.c
@@ -438,6 +438,9 @@ static void __tun_detach(struct tun_file *tfile, bool clean)
sock_put(&tfile->sk);
}
+ if (tun && tun->numqueues == 0)
+ netif_carrier_off(tun->dev);
+
if (clean) {
if (tun && tun->numqueues == 0 && tun->numdisabled == 0 &&
!(tun->flags & TUN_PERSIST))
@@ -473,6 +476,7 @@ static void tun_detach_all(struct net_device *dev)
BUG_ON(tun->numqueues != 0);
synchronize_net();
+ netif_carrier_off(dev);
for (i = 0; i < n; i++) {
tfile = rtnl_dereference(tun->tfiles[i]);
/* Drop read queue */
@@ -531,6 +535,7 @@ static int tun_attach(struct tun_struct *tun, struct file *file)
sock_hold(&tfile->sk);
tun_set_real_num_queues(tun);
+ netif_carrier_on(tun->dev);
/* device is allowed to go away first, so no need to hold extra
* refcnt.
--
1.7.1
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