Homework: xv6 locking

In this assignment you will explore some of the interaction between interrupts and locking. Submit your solutions before the beginning of the next lecture to the submission web site.

Don't do this

Make sure you understand what would happen if the xv6 kernel executed the following code snippet: 

  struct spinlock lk;
  initlock(&lk, "test lock");
  acquire(&lk);
  acquire(&lk);
(Feel free to use QEMU to find out. acquire is in spinlock.c.)

Submit: Explain in one sentence what happens.

Interrupts in ide.c

An acquire ensures that interrupts are off on the local processor using the cli instruction (via pushcli()), and that interrupts remain off until the release of the last lock held by that processor (at which point they are enabled using sti).

Let's see what happens if we turn on interrupts while holding the ide lock. In iderw in ide.c, add a call to sti() after the acquire(), and a call to cli() just before the release(). Rebuild the kernel and boot it in QEMU. Chances are the kernel will panic soon after boot; try booting QEMU a few times if it doesn't.

Submit: Explain in a few sentences why the kernel panicked. You may find it useful to look up the stack trace (the sequence of %eip values printed by panic) in the kernel.asm listing.

Interrupts in file.c

Remove the sti() and cli() you added, rebuild the kernel, and make sure it works again.

Now let's see what happens if we turn on interrupts while holding the file_table_lock. This lock protects the table of file descriptors, which the kernel modifies when an application opens or closes a file. In filealloc() in file.c, add a call to sti() after the call to acquire(), and a cli() just before each of the release()es. You will also need to add #include "x86.h" at the top of the file after the other #include lines. Rebuild the kernel and boot it in QEMU. It will not panic.

Submit: Explain in a few sentences why the kernel didn't panic. Why do file_table_lock and ide_lock have different behavior in this respect?

You do not need to understand anything about the details of the IDE hardware to answer this question, but you may find it helpful to look at which functions acquire each lock, and then at when those functions get called.

(There is a very small but non-zero chance that the kernel will panic with the extra sti() in filealloc(). If the kernel does panic, make doubly sure that you removed the sti() call from iderw. If it continues to panic and the only extra sti() is in filealloc(), then mail 6.828-staff@pdos.csail.mit.edu and think about buying a lottery ticket.)

xv6 lock implementation

Submit: Why does release() clear lk->pcs[0] and lk->cpu before clearing lk->locked? Why not wait until after?