[$] Revocable references for transient devices

Computers were once relatively static devices; if a peripheral was present
at 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 kernel
code, which may not be expecting resources it is managing to make an abrupt
exit. The revocable
resource management patch set from Tzung-Bi Shih is meant to help with
the creation of more robust — and more secure — kernel subsystems in a
dynamic world.