can't unmount unionfs

[Iain Hibbert <plunky%rya-online.net@localhost>]

Hi,
  Updating to -current kernel (as of today, previous was november) and
notice that union mounts are having difficulty unmounting if any files
have been created in the filesystem.

  eg

        mount -t union -o -b /usr/src /usr/plunky/src
        cd /usr/plunky/src
        touch somefile
        cd /usr/plunky
        umount /usr/plunky/src

fails with "Device Busy"

It doesn't always happen but in a simplistic case, it seems that if I do
the above and it fails, then exit the shell and start another to unmount
it will succeed so it seems something is not being flushed. I didn't
update any userland as yet, but UPDATING doesn't mention anything that
would seem related.

.. any ideas?

iain