[$] Lockless patterns: relaxed access and partial memory barriers

The first article in this series provided
an introduction to lockless algorithms and the happens before
relationship that allows us to reason about them. The next step is to look
at the concept of a 'data race' and the primitives that exist to prevent
data races. We continue in that direction with a look at relaxed accesses, memory
barriers, and how they can be used to implement the kernel's seqcount
mechanism.