pre-empt both user and kernel, in clock interrupt

usertest.c tests pre-emption
kill()

Notes

@@ -80,16 +80,22 @@ trap() ought to lgdt on return, since currently only done in swtch()
 
 protect hardware interrupt vectors from user INT instructions?
 
-i'm getting a curious interrupt when jumping into user space. maybe
-it's IRQ 0, but it comes at a weird and changing vector (e.g. 119) if
-you don't initialize the PIC. why doesn't jos see this? if i
-initialize the PIC with IRQ_OFFSET 32, the interrupt arrives at vector
-32.
-
 test out-of-fd cases for creating pipe.
-test pipe circular buffer
-test pipe writer or reader closes while other active or waiting
-test exit vs fd reference counts
-test write of more than PIPESIZE
-test reader goes first vs writer goes first
-test streaming of a lot of data
+test pipe reader closes then write
+test two readers, two writers.
+test children being inherited by grandparent &c
+
+kill
+  sleep()ing for something
+  running at user level
+  running in kernel
+  ooh, the relevant CPU may never get a clock interrupt
+  should each cpu have its own clock?
+  where to check?
+    loops around sleep()
+    return from any trap
+  rules about being killed deep inside a system call
+  test above cases
+
+cli/sti in acquire/release should nest!
+  in case you acquire two locks