For those times when you need to wait for a device to unregister, before you
continue on, this is device_unregister_wait(). It will block until all the
references to the device go away, including references held by open sysfs
files.

This is probably most useful for modules that add and remove devices when
they are loaded and unloaded. They must be sure that the devices that they
have declared or allocated are completely torn down before the module is 
removed, lest unfreed memory get referenced or called.