New comments about thread allocation and deallocation.

kern/thread/thread.c

@@ -255,6 +255,9 @@ cpu_create(unsigned hardware_number)
  * Nor can it be called on a running thread.
  *
  * (Freeing the stack you're actually using to run is ... inadvisable.)
+ *
+ * Thread destroy should finish the process of cleaning up a thread started by
+ * thread_exit.
  */
 static
 void
@@ -263,11 +266,6 @@ thread_destroy(struct thread *thread)
 	KASSERT(thread != curthread);
 	KASSERT(thread->t_state != S_RUN);
 
-	/*
-	 * If you add things to struct thread, be sure to clean them up
-	 * either here or in thread_exit(). (And not both...)
-	 */
-
 	/* Thread subsystem fields */
 	KASSERT(thread->t_proc == NULL);
 	if (thread->t_stack != NULL) {
@@ -771,6 +769,13 @@ thread_startup(void (*entrypoint)(void *data1, unsigned long data2),
  * should be cleaned up right away. The rest has to wait until
  * thread_destroy is called from exorcise().
  *
+ * Note that any dynamically-allocated structures that can vary in size from
+ * thread to thread should be cleaned up here, not in thread_destroy. This is
+ * because the last thread left on each core runs the idle loop and does not
+ * get cleaned up until new threads are created. Differences in the amount of
+ * memory used by different threads after thread_exit will make it look like
+ * your kernel in leaking memory and cause some of the test161 checks to fail.
+ *
  * Does not return.
  */
 void