Well, it's been a year since I last did one of these posts, it seems. Gosh. Time flying.
buffer.c does a lot of looping around the buffer list for nr_buffers * 2 elements. Not explained in any comments. I guess it's an, ahem, heuristic for dealing with race conditions? "free_list" confused me for quite a while, but it's actually more a "freeable" list, containing things that are free or able to be freed.
inode.c is similar, but for inodes rather than buffers.
super.c - yep, superblocks. Reading this code, and the million calls to "iput", I'm so thankful for smart pointers and RAII.
namei.c demonstrates cut-and-paste code for a bunch a of system calls. "unlink" and "rmdir" lack synchronisation, while the others have it. I guess this is because you can't race on named entries when all you're doing is removing them. No explanation in the source, obviously.
block_dev.c is fairly obvious, although the read-ahead infrastructure is nice.
devices.c has a magical default device file operations structure, which just replaces itself with the device-specific operation structure when open is called. That's kinda nutty.
Lessons: I think I keep saying this as I read the code, but C is a horrible language for this kind of thing, and I love RAII to track resources, and I really like Don't Repeat Yourself code. Structs/objects should either be completely initalised, or not at all, so that things are always in a sane state. The use of NULL for "do default action" is crazy. Tony Hoare had the right idea about NULL.