Rusty Russell [interview] recently posted an updated version of his "Unreliable Guide To Locking". The introduction begins:

"Welcome, to Rusty's Remarkably Unreliable Guide to Kernel Locking issues. This document describes the locking systems in the Linux Kernel in 2.6. With the wide availability of HyperThreading, and preemption in the Linux Kernel, everyone hacking on the kernel needs to know the fundamentals of concurrency and locking for SMP. "

Rusty's excellent guide is quite informative, helping the reader to grasp concurrency, explaining the common types of locks, providing useful examples, listing common problems, discussing locking speed, and much more. All in all, it's an essential reference.