To recap my argument: the current cdev implementation keeps an uncounted
reference to every cdev in cdev_map.  Creators of cdevs must know to call
cdev_unmap() with the same arguments they passed to cdev_add() before
releasing the device, or that reference will remain and will oops the
kernel should user space attempt to open the (missing) device.  It's an
easy mistake to make, and, IMO, entirely unnecessary; the cdev code should
be able to do its own bookkeeping.