umount doesn't care about vnode references or open files

[OlliV]

Umount will hapily unmount a super block despite of any open files or references to vnodes. This behavior may cause instability or crashes in the kernel.

Counting open files would solve the issue for open files. The second case is however more tricky because if a vnode is referenced by the kernel it's not usually considered as an open file, thus counting open files doesn't solve the issue for any kernel references (eg. device files).