[Lxc-users] Zombie container

[Milan Zamazal]

On a Debian 6.0 machine, I've got a certain container that can't be
started again once it was stopped:

  # lxc-start -n blackbird
  lxc-start: Device or resource busy - failed to remove previous cgroup '/cgroup/blackbird'
  lxc-start: failed to spawn 'blackbird'
  lxc-start: Device or resource busy - failed to remove cgroup '/cgroup/blackbird'

The container seems to be stopped completely but the /cgroup/blackbird/
directory is indeed non-empty.  There are some subdirectories with
numeric names there but I can't find any processes with such numbers in
the system nor any other processes related to the container.  The only
way to get rid of it is to reboot the host.

Is there a way to force removal of the cgroup?  Or is there a way to
find out what keeps the cgroup busy?