Read Copy Update (aka RCU) is another synchronisation mechanism in order to avoid reader writer locks.
An excellent explanation can be found at the LWN.net in three parts by Paul McKenney and Jonathan Walpole:
The basic idea behind it is that when a resource is modified, a new updated structure is put in its place and the old structure is not discarded right away, it waits until references to this structure by other processes are dropped. It can be seen as similar to the concept of garbage collection, but as noted in What is RCU? Part 2: usage, the old structure is not discarded automatically when there are no references any more and the programmer must indicate the critical read portions of the code.
There is an interesting page on the RCU argueing that this technique is used more and more in the kernel as a replacement for the reader writer locks.
