Comment by nickpsecurity

masklinn's claim still rings true. There's a difference between codebases designed mostly for being easy to understand and codebases designed to be optimal in performance, consistency with an existing style (esp if old), and portability. Getting used to reading one might not transfer well to the other kind since the patterns are different. Probably best to use one of its intended goal (learning how OS's work) and look at examples of the other kind when trying to absorb patterns seen in real-world codebases.