What parts of Linux kernel can I read for fun?

Question

Programming isn't my main job, though I enjoy it and sometimes get paid for it. For many years now I've been hearing about Linux and my friends have shown to me many *nixes (or *nici?), though I stick with Mac OS.

Do you think there are any parts of the Linux kernel that I could enjoy looking at, that would help me understand what's the whole stuff about? For example, how Linux is different from Darwin?

I've grown up with assembler and DOS, so things like interrupts or low-level C shouldn't be barriers to understanding. But in the end I'm more interested in high-level concepts, like threading or networking stack - I know different operating systems do them differently. And I'm looking for something fun, easy and enjoyable, like late-night reading.

(Note: made a CW, just in case)

Update: I looked for some docs and started reading:

• Unreliable Guide To Locking

Answer 1

I would recommend looking at LXR. It makes it easier to follow the flow of the code (you do not have to search for each function that is called — well, you have, but the site does it for you).

Some starting points, for the current version (2.6.30):

• start_kernel() — think of it as the kernel equivalent of main(). This function initializes almost all the kernel subsystems; follow it to see in code what you see scrolling on the screen during the boot.
• entry_32.S — system calls and interrupts (x86-32 version, which should be nearer what you know; note the use of the AT&T assembly dialect instead of the Intel dialect you might be more used to).
• head_32.S — the kernel entry point. This is where the kernel starts after switching to protected mode; in the end, it will call start_kernel().
• arch/x86/boot — the real-mode bootstrap code. It starts in assembly (boot/header.S), but quickly jumps into C code (starting at boot/main.c). Does the real-mode initialization (mostly BIOS calls which have to be done before switching to protected mode); it is compiled using a weird GCC trick (.code16gcc), which allows the generation of 32-bit real-mode code.
• arch/x86/boot/compressed — if you ever wondered where does the "Decompressing Linux..." message comes from, it is from here.