Have you actually ever read any of the Linux source code? I find it pretty hard to read to be honest. There are barely any comments and the ones that are there are often not very helpful.

You'd have to explain to me what you think makes the Linux code of particularly high quality; I don't think it's a good fit for the criteria in the question.

Whilst it may well well engineered as a project of this size, the code I have seen leaves a lot to be desired.

Cases that spring to mind are the mm code and cgroups where, as an outsider, I got the impression of a large codebase where the patch delta is kept to a minimum -- perhaps so as not to introduce new bugs as features are added. But this doesn't necessarily mean the resulting code has a logical structure or layering; it means deciphering it involves as much studying of the Git history as it does of the (sparsely commented) code on the screen.

In contrast, my experiences in the FreeBSD code have been much better, with less spaghetti and even to the point of maintained man pages for key internal functions.