--- a/hotspot/src/share/vm/gc_implementation/g1/concurrentMark.cpp	Fri Feb 01 17:21:53 2013 -0800
+++ b/hotspot/src/share/vm/gc_implementation/g1/concurrentMark.cpp	Thu Jan 31 10:45:09 2013 -0800
@@ -1190,7 +1190,7 @@
     uint active_workers = MAX2(1U, parallel_marking_threads());
 
     CMRootRegionScanTask task(this);
-    if (parallel_marking_threads() > 0) {
+    if (use_parallel_marking_threads()) {
       _parallel_workers->set_active_workers((int) active_workers);
       _parallel_workers->run_task(&task);
     } else {
@@ -1226,7 +1226,7 @@
   set_phase(active_workers, true /* concurrent */);
 
   CMConcurrentMarkingTask markingTask(this, cmThread());
-  if (parallel_marking_threads() > 0) {
+  if (use_parallel_marking_threads()) {
     _parallel_workers->set_active_workers((int)active_workers);
     // Don't set _n_par_threads because it affects MT in proceess_strong_roots()
     // and the decisions on that MT processing is made elsewhere.
@@ -3242,7 +3242,9 @@
 }
 
 void ConcurrentMark::print_worker_threads_on(outputStream* st) const {
-  _parallel_workers->print_worker_threads_on(st);
+  if (use_parallel_marking_threads()) {
+    _parallel_workers->print_worker_threads_on(st);
+  }
 }
 
 // We take a break if someone is trying to stop the world.

--- a/hotspot/src/share/vm/gc_implementation/g1/concurrentMark.hpp	Fri Feb 01 17:21:53 2013 -0800
+++ b/hotspot/src/share/vm/gc_implementation/g1/concurrentMark.hpp	Thu Jan 31 10:45:09 2013 -0800
@@ -499,17 +499,26 @@
   }
 
   // accessor methods
-  uint parallel_marking_threads() { return _parallel_marking_threads; }
-  uint max_parallel_marking_threads() { return _max_parallel_marking_threads;}
-  double sleep_factor()             { return _sleep_factor; }
-  double marking_task_overhead()    { return _marking_task_overhead;}
-  double cleanup_sleep_factor()     { return _cleanup_sleep_factor; }
-  double cleanup_task_overhead()    { return _cleanup_task_overhead;}
+  uint parallel_marking_threads() const     { return _parallel_marking_threads; }
+  uint max_parallel_marking_threads() const { return _max_parallel_marking_threads;}
+  double sleep_factor()                     { return _sleep_factor; }
+  double marking_task_overhead()            { return _marking_task_overhead;}
+  double cleanup_sleep_factor()             { return _cleanup_sleep_factor; }
+  double cleanup_task_overhead()            { return _cleanup_task_overhead;}
 
-  HeapWord*               finger()        { return _finger;   }
-  bool                    concurrent()    { return _concurrent; }
-  uint                    active_tasks()  { return _active_tasks; }
-  ParallelTaskTerminator* terminator()    { return &_terminator; }
+  bool use_parallel_marking_threads() const {
+    assert(parallel_marking_threads() <=
+           max_parallel_marking_threads(), "sanity");
+    assert((_parallel_workers == NULL && parallel_marking_threads() == 0) ||
+           parallel_marking_threads() > 0,
+           "parallel workers not set up correctly");
+    return _parallel_workers != NULL;
+  }
+
+  HeapWord*               finger()          { return _finger;   }
+  bool                    concurrent()      { return _concurrent; }
+  uint                    active_tasks()    { return _active_tasks; }
+  ParallelTaskTerminator* terminator()      { return &_terminator; }
 
   // It claims the next available region to be scanned by a marking
   // task/thread. It might return NULL if the next region is empty or