What is "read-copy-update"

In computer operating systems, read-copy-update (RCU) is a synchronization mechanism implementing a kind of mutual exclusion that can sometimes be used as an alternative to a readers-writer lock. It allows extremely low overhead, wait-free reads. However, RCU updates can be expensive, as they must leave the old versions of the data structure in place to accommodate pre-existing readers. These old versions are reclaimed after all pre-existing readers finish their accesses.