[$] The RCU API, 2019 edition

Read-copy update (RCU) is a synchronization mechanism that was added to
the 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 also
permits RCU readers to accomplish useful work even
when running concurrently with updaters.
Although the basic idea behind RCU has not changed in
decades following its introduction into DYNIX/ptx, the API has
evolved significantly over the five years since the
2014 edition of the RCU API,
to say nothing of the nine years since the
2010 edition of the RCU API.