any help for zfs on linux continuing inability to unmount zfs file systems

[Harry Putnam]

Some may be familiar with this problem if using zfs on ubuntu. That is, not
being able to unmount a zfs file system... getting the message
". . target is busy".

Googling I see this probem has been with us for a few yrs but yet I'm
not finding an helpful information ... just other users having the
same problem.

Can anyone offer some help on this?