Abstract: Read-copy update (RCU) is a scalable high-performance synchronization mechanism implemented in the Linux kernel. RCU's novel properties include support for concurrent forward progress for readers and writers as well as highly optimized inter-CPU synchronization. RCU was introduced into the Linux kernel eighteen years ago and most subsystems now use RCU. This paper discusses the requirements that drove the development of RCU, the design and API of the Linux RCU implementation, and how kernel developers apply RCU.