When running `umount /path` I get:

 umount: /path: device is busy.

The filesystem is huge, so `lsof +D /path` is not a realistic option.

`lsof /path`, `lsof +f -- /path`, and `fuser /path` all return nothing. `fuser -v /path` gives:

 USER PID ACCESS COMMAND
 /path: root kernel mount /path

which is normal for all unused mounted file systems.

`umount -l` and `umount -f` is not good enough for my situation.

How do I figure out why the kernel thinks this filesystem is busy?

I have made a page with examples of all solutions so far here:
http://oletange.blogspot.com/2012/04/umount-device-is-busy-why.html