So, my reading of the kernel code ends not with a bang but with a whimper. tcp.c is the last source file to read, and is a 3.7kloc mess. Most of the code looks plausible, but would take far too much time to understand if it's doing the right things. tcp_ack is pretty horrible. tcp_rcv is too. The structure make responsibilities unclear (not helped by C's tendency to mix the lowest-level details with the highest level abstractions). I guess one could charitably say it's compact, but it's in the "so complex there's no obvious bugs" category. To be frank, I just skimmed it.
There are various things I've learnt reading this kernel. One is that a clear and consistent programming model is necessary if you want a maintainable kernel (e.g. around how to avoid race conditions, how to handle operation completions, etc.). Another is that C is a really bad language for this kind of thing, and RAII-like tooling is necessary to stop resource leaks being risky. Clear abstractions are great, good comments are important, and life's too short to read hard-to-read code.