--- a/hotspot/src/share/vm/gc_implementation/concurrentMarkSweep/concurrentMarkSweepGeneration.cpp	Wed Jan 20 12:54:25 2010 -0800
+++ b/hotspot/src/share/vm/gc_implementation/concurrentMarkSweep/concurrentMarkSweepGeneration.cpp	Thu Jan 21 14:47:01 2010 -0800
@@ -3655,9 +3655,7 @@
   verify_work_stacks_empty();
   verify_overflow_empty();
   assert(_revisitStack.isEmpty(), "tabula rasa");
-
-  DEBUG_ONLY(RememberKlassesChecker cmx(CMSClassUnloadingEnabled);)
-
+  DEBUG_ONLY(RememberKlassesChecker cmx(should_unload_classes());)
   bool result = false;
   if (CMSConcurrentMTEnabled && ParallelCMSThreads > 0) {
     result = do_marking_mt(asynch);
@@ -4124,7 +4122,6 @@
 void CMSConcMarkingTask::coordinator_yield() {
   assert(ConcurrentMarkSweepThread::cms_thread_has_cms_token(),
          "CMS thread should hold CMS token");
-
   DEBUG_ONLY(RememberKlassesChecker mux(false);)
   // First give up the locks, then yield, then re-lock
   // We should probably use a constructor/destructor idiom to
@@ -4201,9 +4198,7 @@
   // Mutate the Refs discovery so it is MT during the
   // multi-threaded marking phase.
   ReferenceProcessorMTMutator mt(ref_processor(), num_workers > 1);
-
-  DEBUG_ONLY(RememberKlassesChecker cmx(CMSClassUnloadingEnabled);)
-
+  DEBUG_ONLY(RememberKlassesChecker cmx(should_unload_classes());)
   conc_workers()->start_task(&tsk);
   while (tsk.yielded()) {
     tsk.coordinator_yield();
@@ -4472,7 +4467,7 @@
     // for cleaner interfaces.
     rp->preclean_discovered_references(
           rp->is_alive_non_header(), &keep_alive, &complete_trace,
-          &yield_cl);
+          &yield_cl, should_unload_classes());
   }
 
   if (clean_survivor) {  // preclean the active survivor space(s)
@@ -4494,7 +4489,7 @@
     SurvivorSpacePrecleanClosure
       sss_cl(this, _span, &_markBitMap, &_markStack,
              &pam_cl, before_count, CMSYield);
-    DEBUG_ONLY(RememberKlassesChecker mx(CMSClassUnloadingEnabled);)
+    DEBUG_ONLY(RememberKlassesChecker mx(should_unload_classes());)
     dng->from()->object_iterate_careful(&sss_cl);
     dng->to()->object_iterate_careful(&sss_cl);
   }
@@ -4665,7 +4660,7 @@
         verify_work_stacks_empty();
         verify_overflow_empty();
         sample_eden();
-        DEBUG_ONLY(RememberKlassesChecker mx(CMSClassUnloadingEnabled);)
+        DEBUG_ONLY(RememberKlassesChecker mx(should_unload_classes());)
         stop_point =
           gen->cmsSpace()->object_iterate_careful_m(dirtyRegion, cl);
       }
@@ -4753,7 +4748,7 @@
       sample_eden();
       verify_work_stacks_empty();
       verify_overflow_empty();
-      DEBUG_ONLY(RememberKlassesChecker mx(CMSClassUnloadingEnabled);)
+      DEBUG_ONLY(RememberKlassesChecker mx(should_unload_classes());)
       HeapWord* stop_point =
         gen->cmsSpace()->object_iterate_careful_m(dirtyRegion, cl);
       if (stop_point != NULL) {
@@ -4853,7 +4848,7 @@
   assert(haveFreelistLocks(), "must have free list locks");
   assert_lock_strong(bitMapLock());
 
-  DEBUG_ONLY(RememberKlassesChecker fmx(CMSClassUnloadingEnabled);)
+  DEBUG_ONLY(RememberKlassesChecker fmx(should_unload_classes());)
   if (!init_mark_was_synchronous) {
     // We might assume that we need not fill TLAB's when
     // CMSScavengeBeforeRemark is set, because we may have just done

--- a/hotspot/src/share/vm/memory/iterator.hpp	Wed Jan 20 12:54:25 2010 -0800
+++ b/hotspot/src/share/vm/memory/iterator.hpp	Thu Jan 21 14:47:01 2010 -0800
@@ -296,23 +296,32 @@
 // RememberKlassesChecker can be passed "false" to turn off checking.
 // It is used by CMS when CMS yields to a different collector.
 class RememberKlassesChecker: StackObj {
- bool _state;
- bool _skip;
+ bool _saved_state;
+ bool _do_check;
  public:
-  RememberKlassesChecker(bool checking_on) : _state(false), _skip(false) {
-    _skip = !(ClassUnloading && !UseConcMarkSweepGC ||
-              CMSClassUnloadingEnabled && UseConcMarkSweepGC);
-    if (_skip) {
-      return;
+  RememberKlassesChecker(bool checking_on) : _saved_state(false),
+    _do_check(true) {
+    // The ClassUnloading unloading flag affects the collectors except
+    // for CMS.
+    // CMS unloads classes if CMSClassUnloadingEnabled is true or
+    // if ExplicitGCInvokesConcurrentAndUnloadsClasses is true and
+    // the current collection is an explicit collection.  Turning
+    // on the checking in general for
+    // ExplicitGCInvokesConcurrentAndUnloadsClasses and
+    // UseConcMarkSweepGC should not lead to false positives.
+    _do_check =
+      ClassUnloading && !UseConcMarkSweepGC ||
+      CMSClassUnloadingEnabled && UseConcMarkSweepGC ||
+      ExplicitGCInvokesConcurrentAndUnloadsClasses && UseConcMarkSweepGC;
+    if (_do_check) {
+      _saved_state = OopClosure::must_remember_klasses();
+      OopClosure::set_must_remember_klasses(checking_on);
     }
-    _state = OopClosure::must_remember_klasses();
-    OopClosure::set_must_remember_klasses(checking_on);
   }
   ~RememberKlassesChecker() {
-    if (_skip) {
-      return;
+    if (_do_check) {
+      OopClosure::set_must_remember_klasses(_saved_state);
     }
-    OopClosure::set_must_remember_klasses(_state);
   }
 };
 #endif  // ASSERT

--- a/hotspot/src/share/vm/memory/referenceProcessor.cpp	Wed Jan 20 12:54:25 2010 -0800
+++ b/hotspot/src/share/vm/memory/referenceProcessor.cpp	Thu Jan 21 14:47:01 2010 -0800
@@ -1227,13 +1227,16 @@
   BoolObjectClosure* is_alive,
   OopClosure* keep_alive,
   VoidClosure* complete_gc,
-  YieldClosure* yield) {
+  YieldClosure* yield,
+  bool should_unload_classes) {
 
   NOT_PRODUCT(verify_ok_to_handle_reflists());
 
 #ifdef ASSERT
   bool must_remember_klasses = ClassUnloading && !UseConcMarkSweepGC ||
-                               CMSClassUnloadingEnabled && UseConcMarkSweepGC;
+                               CMSClassUnloadingEnabled && UseConcMarkSweepGC ||
+                               ExplicitGCInvokesConcurrentAndUnloadsClasses &&
+                                 UseConcMarkSweepGC && should_unload_classes;
   RememberKlassesChecker mx(must_remember_klasses);
 #endif
   // Soft references

--- a/hotspot/src/share/vm/memory/referenceProcessor.hpp	Wed Jan 20 12:54:25 2010 -0800
+++ b/hotspot/src/share/vm/memory/referenceProcessor.hpp	Thu Jan 21 14:47:01 2010 -0800
@@ -170,11 +170,13 @@
   // The caller is responsible for taking care of potential
   // interference with concurrent operations on these lists
   // (or predicates involved) by other threads. Currently
-  // only used by the CMS collector.
+  // only used by the CMS collector.  should_unload_classes is
+  // used to aid assertion checking when classes are collected.
   void preclean_discovered_references(BoolObjectClosure* is_alive,
                                       OopClosure*        keep_alive,
                                       VoidClosure*       complete_gc,
-                                      YieldClosure*      yield);
+                                      YieldClosure*      yield,
+                                      bool               should_unload_classes);
 
   // Delete entries in the discovered lists that have
   // either a null referent or are not active. Such