I've been reading more source for a while, but let's catch up on the write-up. In this post, I'll cover the remainder of the non-fs-type-specific code from the fs/ directory.
You may notice my descriptions getting terse as I get a bit bored of the same old stuff, at this point.
pipe.c and fifo.c implement pipes, as you might guess. fcntl.c is boring. locks.c is very well-commented. Perhaps even overly-commented! It doesn't stop "lock_it" being hairy, and I'm too lazy to read it properly.
open.c actually contains a bunch of inode-twiddling syscalls. It's a bit sad to see the level of C&P between the sys_XXX and sys_fXXX syscalls. read_write.c is just a few syscall wrappers around the underlying functionality. select.c... well, it shows you how the select system call is implemented over the kernel waiting mechanism.
Finally, stat.c is a surprisingly boring set of syscalls to end off with.
At this point, I believe I've read all of the "core" OS. What remains is device drivers, filesystem implementations and the networking infrastructure. As tends to be the way with these things, the majority of the code is in these peripheral parts. So, plenty to go.