[$] Hazard pointers for the kernel

The kernel's read-copy-update (RCU) subsystem ensures that data will not bedeleted until it is known that there are no threads holding references toit. RCU works well and is widely used throughout the kernel, but it canincrease memory use and add significant delays before unused kernel objectsare cleaned up. Hazard pointers arean alternative approach to lockless data updates that offers betterperformance, for some situations at least. The kernel community iscurrently considering ahazard-pointer implementation by Mathieu Desnoyers and Paul McKenney.