[$] Revocable references for transient devices

Computers were once relatively static devices; if a peripheral was presentat boot, it was unlikely to disappear while the system was operating.Those days are far behind us, though; devices can come and go at any time,often with no notice. That impermanence can create challenges for kernelcode, which may not be expecting resources it is managing to make an abruptexit. The revocableresource management patch set from Tzung-Bi Shih is meant to help withthe creation of more robust - and more secure - kernel subsystems in adynamic world.