2.5.59 NFS server keeps local fs live after being stopped

[Mikael Pettersson]

Kernel 2.5.59. A local ext2 file system is mounted at $MNTPNT
and exported through NFS V3. A client mounts and unmounts it,
w/o any I/O in between. The NFS server is shut down. Nothing in
user-space refers to $MNTPNT.

The bug is that $MNTPNT now can't be unmounted. umount fails with
"device is busy". A forced umount at shutdown fails with "device
or resource busy" and "illegal seek", and leaves the underlying
fs marked dirty.

I can't say exactly when this began, but the problem is present
in 2.5.59 and 2.5.55. 2.4.21-pre4 does not have this problem.

/Mikael