When some code section happens to be called from both thread context and
interrupt context, and we need mutual exclusion (we don't want the interrupt
context to start while the critical section is in the middle of running in
thread context), we surround the critical code section with CLI and STI.

But we need the compiler to assure us that writes to memory done between
the calls to CLI and STI stay between them. For example, if we have

    thread context:                 interrupt handler:

      CLI;                          a--;
      a++;
      STI;

We don't want the a++ to be moved by the compiler before the CLI. We also
don't want the compiler to save a's value in a register and only actually
write it back to the memory location 'a' after the STI (when an interrupt
handler might be concurrently writing). We also don't want the compiler
to remember a's last value in a register and use it again after the next
CLI.

To ensure these things, we need the "memory clobber" option on both the CLI
and STI instructions. The "volatile" keyword is not enough - it guarantees
that the instruction isn't deleted or moved, but not that stuff that
should have been in memory isn't just in registers.

Note that Linux also has these memory clobbers on sti() and cli().
Linus Torvals explains in a post from 1996 why these were necessary:
http://lkml.indiana.edu/hypermail/linux/kernel/9605/0214.html

All that being said, we never noticed a bug caused by the missing
"memory" clobbers. But better safe than sorry....