+/* stopping the world is a two-stage process. From this thread we signal
+ * all the others with SIG_STOP_FOR_GC. The handler for this thread does
+ * the usual pseudo-atomic checks (we don't want to stop a thread while
+ * it's in the middle of allocation) then kills _itself_ with SIGSTOP.
+ * At any given time, countdown_to_gc should reflect the number of threads
+ * signalled but which haven't yet come to rest
+ */
+