[perfbook] Possible improvements of Figures 9.11 -- 9.13

[Akira Yokosawa <akiyks@xxxxxxxxx>]

Hi Paul,

I'm reviewing Section 9.5.
I find Figures 9.11 -- 9.13 are somewhat hard to grasp.

In those figures, P0() is called "RCU Reader" due to the
RCU read-side critical section.

However, it updates x and y, and you are talking about
final values of x and y.

At first glance, it looked as though P0() would read from
x and y, and get values 1 or 2.

Some clarification would help new readers of Edition 2,
I suppose.

        Thanks, Akira