Ok, I've got a minimal reproducer ready.  It causes the bug when I run
it on Ubuntu 14.04 with 3.13.0-24-generic and LXC version
1.0.7-0ubuntu0.1 0.  It's python, and the script has some documentation
describing its usage as well as the prerequisites.  You should run it as
root via sudo.

Prereqs require you to install a base ubuntu trusty container, install
nfs-common in that container, and modify lxc-default to allow containers
to perform NFS mounts.  You will also need an NFS share that your
containers can mount to scribble data to.  The address of this share is
passed as the third parameter to the script in the form of
IP_ADDRESS:/path/to/nfs/share.

My typical usage looks like:

sudo ./reproducer.py 5 10 IPADDRESS:/path/to/nfs/share

Which starts 5 threads that in each of 10 iterations, creates a
container, mounts the nfs share to /mnt, then dd's over some zeros,
umounts /mnt, stops, and destroys the container.  I can reliably hit the
unregister_netdevice hang within 1 minute of running this.

~Dave

** Attachment added: "Script to reproduce the bug"
   
https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1403152/+attachment/4391814/+files/reproducer.py

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Title:
  unregister_netdevice: waiting for lo to become free. Usage count

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