Read-copy update

Traditional operating-system locking designs tend to either be very complex, result in poor concur-rency, or both. These traditional locking designs fail to take advantage of the event-driven nature of op-erating systems, which process many small, quickly completed units of work, in contrast to CPU-bound software such as scientic applications. This event-driven nature can often be exploited by splitting up-dates into the two phases: 1) carrying out enough of each update for new operations to see the new state, while still allowing existing operations to proceed on the old state, then: 2) completing the update after all active operations have completed. Common-case code can then proceed without disabling interrupts or acquiring any locks to protect against the update code, which simplies locking protocols, improves uniprocessor performance, and increases scalability. Examples of the application of these techniques in-clude maintaining read-mostly data structures, such as routing tables, avoiding the need for existence locks (and hence avoiding locking hierarchies with the attendant deadlock issues), and dealing with un-usual situations like module unloading.