Re: NET_NS: unregister_netdevice: waiting for lo to become free (adding ipv6 address to interface)

On Thu, 05 Aug 2010 02:51:29 -0700
ebiederm@xmission.com (Eric W. Biederman) wrote:

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Jul 10 20:02:36 doris kernel: unregister_netdevice: waiting for
lo to become free. Usage count = 3 [repeated]

How many times?

Unfortunately looks like indefinitely. Never watched longer so far
(rebooted soon), but I'm seeing this message now repeated every 10
secs for ~10 minutes on a idle system.

Ugh.  A real bug then.  These can be a pain to track down and fix. I
think the last one of these I tracked down took a couple of weeks.  I
will start digging in when I get back from vacation.

OK, fortunately (hopefully) you have not put to much time onto that so
far - because everything I told about usage of tun and difference
between ssh and openvpn is complete nonsense.

I happen to have an script in that openvpn config, which puts an ipv6
address on the vpn device.

Putting an ipv6 address on a device seems to be the trigger:

OrigNS > # ip link add type veth
OrigNS > # ip link set dev veth0 up
OrigNS > # unshare -n /bin/bash
NewNS > # echo $$
<SomePID>
OrigNS > # ip link set dev veth1 netns <SomePID> # this, of course is on a different terminal
NewNS > # ip link set dev veth1 up
NewNS > # ip -6 addr add dev veth1 fd50:dead:beef::1/64
NewNS > # exit

Yields

kernel: unregister_netdevice: waiting for veth1 to become free. Usage count = 3

Oh - its veth1 this time, not lo - add an "ip link set up dev lo" in the above scenario just after the unshare, and you get the message with lo.

One might ask, if

# unshare -n /bin/bash
# ip link set up dev lo
# ip -6 addr add dev veth1 fd50:dead:beef::1/64
# exit

also does the trick, so I tried it - and it does NOT.

In the above scenario, not setting veth0 and veth1 up also makes it not happen. Only setting veth1 up also is not enough (seems to need to be "really up" what as you shurely know with veth is only the case when both sides are up).

I hope, this makes it somewhat easier to track that down.

-- 
MfG,

Michael Leun