diff -r 28d3809ff91f -r 307e3d07d19f src/Pure/Concurrent/task_queue.ML
--- a/src/Pure/Concurrent/task_queue.ML	Fri Sep 10 12:39:20 2010 +0200
+++ b/src/Pure/Concurrent/task_queue.ML	Fri Sep 10 14:54:08 2010 +0200
@@ -27,6 +27,7 @@
   val enqueue_passive: group -> (unit -> bool) -> queue -> task * queue
   val enqueue: group -> task list -> int -> (bool -> bool) -> queue -> (task * bool) * queue
   val extend: task -> (bool -> bool) -> queue -> queue option
+  val dequeue_passive: Thread.thread -> task -> queue -> bool * queue
   val dequeue: Thread.thread -> queue -> (task * group * (bool -> bool) list) option * queue
   val depend: task -> task list -> queue -> queue
   val dequeue_towards: Thread.thread -> task list -> queue ->
@@ -216,12 +217,19 @@
 
 (* dequeue *)
 
+fun dequeue_passive thread task (queue as Queue {groups, jobs}) =
+  (case try (get_job jobs) task of
+    SOME (Passive _) =>
+      let val jobs' = set_job task (Running thread) jobs
+      in (true, make_queue groups jobs') end
+  | _ => (false, queue));
+
 fun dequeue thread (queue as Queue {groups, jobs}) =
   (case Task_Graph.get_first (uncurry ready_job) jobs of
-    NONE => (NONE, queue)
-  | SOME (result as (task, _, _)) =>
+    SOME (result as (task, _, _)) =>
       let val jobs' = set_job task (Running thread) jobs
-      in (SOME result, make_queue groups jobs') end);
+      in (SOME result, make_queue groups jobs') end
+  | NONE => (NONE, queue));
 
 
 (* dequeue_towards -- adhoc dependencies *)