so fast interrupts overflow the kernel stack
fix: cli() before lapic_eoi()

wakeup1() assumes you hold proc_table_lock
sleep(chan, lock) provides atomic sleep-and-release to wait for condition
ugly code in swtch/scheduler to implement new sleep
fix lots of bugs in pipes, wait, and exit
fix bugs if timer interrupt goes off in schedule()
console locks per line, not per byte

succeeds at usertests.c pipe test

swtch idles on per-CPU stack, not on calling process's stack
fix pipe bugs
usertest.c tests pipes, fork, exit, close

pipes