If you fine tuned ChatGPT on all the sources you mention, you now have a model that produces results that could plausibly have been written by a domain expert on the Linux kernel, but you don't have a domain expert. It will still hallucinate, because that's a fundamental feature of generative AI, it will just hallucinate much more convincingly.
For example, in my experience ChatGPT isn't very "smart". It has a lot of knowledge but it can't infer any facts from that knowledge. When you ask it to write a program it has no real idea of what it actually does, and you can easily get it to add features it already added, or tell it something is a bug when it really isn't.
This doesn't sound like the stuff you could make a domain expert out of, at least, not out of the box.
"How does common subexpression elimination work?"
It answered it correctly and gave some basic code examples to demonstrate the concept. Then I followed up with:
"But can it do the elimination if the variables are flipped, but semantically equivalent, like 'y + x' in the example above?"
It again gave what I would consider a correct answer. Note 'x + y' was what was eliminated previously, so I reversed it here.
Then I asked: "Are there cases where the compiler might fail to eliminate common subexpressions?"
And again a good answer.
Now all of this could be found on the web somewhere, but for example the second question didn't show an obvious answer on Google when I searched for it. I'm sure I could find it, but I know this field well. If I was someone new to the field I'd probably spend a lot of time parsing useless articles to find something that answered the question in a way I could understand.
I'm less concerned about it writing code (which is cool). For me, the ability to help me learn an area quickly is far more useful. It doesn't need novel answers, but the ability to understand what I'm asking and answer it. I think it's really close to being able to do this now.
For example if you took all the Linux kernel code, the code review comments, the docs, and several of the top books and blogs on kernel development — suddenly you have a system that may be great for new kernel developers to ask questions of. Especially in a community that often isn’t kind to people Jose asking “dumb questions”.