[$] The RCU API, 2019 edition

Read-copy update (RCU) is a synchronization mechanism that was added tothe Linux kernel in October 2002.RCU is most frequently described as a replacement for reader-writer locking,but has also been used in a number of other ways.RCU is notable in that readers do not directly synchronize with updaters,which makes RCU read paths extremely fast; that alsopermits RCU readers to accomplish useful work evenwhen running concurrently with updaters.Although the basic idea behind RCU has not changed indecades following its introduction into DYNIX/ptx, the API hasevolved significantly over the five years since the2014 edition of the RCU API,to say nothing of the nine years since the2010 edition of the RCU API.