Define code quality: braindead GNU extensions such as nested functions - Linux uses that, How about the separation of DM and MD due to ego's, how every driver without a lib (such as libata) warranting a few exceptions has to rewrite boilerplate for basic things - frame buffer drivers are a good example. How about the syscall bugs - look into glibc if you feel like gouging your eyes out or jemalloc for more tame code. Linux itself is more an ecosystem of code and not a single project some parts are good while others are very very bad.