# HG changeset patch
# Parent b9787f124d93c1f71194b2485d2996f92fee80d5
diff --git a/src/hotspot/share/gc/shenandoah/shenandoahConcurrentMark.cpp b/src/hotspot/share/gc/shenandoah/shenandoahConcurrentMark.cpp
--- a/src/hotspot/share/gc/shenandoah/shenandoahConcurrentMark.cpp
+++ b/src/hotspot/share/gc/shenandoah/shenandoahConcurrentMark.cpp
@@ -638,9 +638,9 @@
 
     ShenandoahHeap* heap = ShenandoahHeap::heap();
     ShenandoahConcurrentMark* cm = heap->concurrentMark();
-    ShenandoahPushWorkerScope scope(_workers,
-                                    ergo_workers,
-                                    /* do_check = */ false);
+    ShenandoahPushWorkerQueuesScope scope(_workers, cm->task_queues(),
+                                          ergo_workers,
+                                          /* do_check = */ false);
     uint nworkers = _workers->active_workers();
     cm->task_queues()->reserve(nworkers);
     if (UseShenandoahOWST) {
@@ -722,6 +722,9 @@
 
   ReferenceProcessorPhaseTimes pt(sh->gc_timer(), rp->num_queues());
 
+  // Prepare for single-threaded mode
+  ShenandoahPushWorkerQueuesScope scope(workers, task_queues(), 1, /* do_check = */ false);
+
   {
     ShenandoahGCPhase phase(phase_process);
     ShenandoahTerminationTracker phase_term(phase_process_termination);
@@ -942,7 +945,7 @@
    * with this worker id, we come back to process this queue in the normal loop.
    */
   assert(queues->get_reserved() == heap->workers()->active_workers(),
-    "Need to reserve proper number of queues");
+         "Need to reserve proper number of queues: reserved: %u, active: %u", queues->get_reserved(), heap->workers()->active_workers());
 
   q = queues->claim_next();
   while (q != NULL) {
diff --git a/src/hotspot/share/gc/shenandoah/shenandoahTraversalGC.cpp b/src/hotspot/share/gc/shenandoah/shenandoahTraversalGC.cpp
--- a/src/hotspot/share/gc/shenandoah/shenandoahTraversalGC.cpp
+++ b/src/hotspot/share/gc/shenandoah/shenandoahTraversalGC.cpp
@@ -1093,9 +1093,10 @@
 
     ShenandoahHeap* heap = ShenandoahHeap::heap();
     ShenandoahTraversalGC* traversal_gc = heap->traversal_gc();
-    ShenandoahPushWorkerScope scope(_workers,
-                                    ergo_workers,
-                                    /* do_check = */ false);
+    ShenandoahPushWorkerQueuesScope scope(_workers,
+                                          traversal_gc->task_queues(),
+                                          ergo_workers,
+                                          /* do_check = */ false);
     uint nworkers = _workers->active_workers();
     traversal_gc->task_queues()->reserve(nworkers);
     if (UseShenandoahOWST) {
@@ -1143,6 +1144,8 @@
   ShenandoahTraversalRefProcTaskExecutor executor(workers);
 
   ReferenceProcessorPhaseTimes pt(sh->gc_timer(), rp->num_queues());
+  // Prepare for single-threaded mode
+  ShenandoahPushWorkerQueuesScope scope(workers, task_queues(), 1, /* do_check = */ false);
 
   {
     ShenandoahGCPhase phase(phase_process);
diff --git a/src/hotspot/share/gc/shenandoah/shenandoahWorkGroup.cpp b/src/hotspot/share/gc/shenandoah/shenandoahWorkGroup.cpp
--- a/src/hotspot/share/gc/shenandoah/shenandoahWorkGroup.cpp
+++ b/src/hotspot/share/gc/shenandoah/shenandoahWorkGroup.cpp
@@ -26,6 +26,7 @@
 #include "gc/shenandoah/shenandoahHeap.hpp"
 #include "gc/shenandoah/shenandoahThreadLocalData.hpp"
 #include "gc/shenandoah/shenandoahWorkGroup.hpp"
+#include "gc/shenandoah/shenandoahTaskqueue.hpp"
 
 #include "logging/log.hpp"
 
@@ -61,6 +62,25 @@
   _workers->update_active_workers(_old_workers);
 }
 
+ShenandoahPushWorkerQueuesScope::ShenandoahPushWorkerQueuesScope(WorkGang* workers, ShenandoahObjToScanQueueSet* queues, uint nworkers, bool check) :
+  _workers(workers), _old_workers(workers->active_workers()),
+  _queues(queues), _n_workers(nworkers) {
+  _workers->update_active_workers(nworkers);
+  _queues->reserve(nworkers);
+  // bypass concurrent/parallel protocol check for non-regular paths, e.g. verifier, etc.
+  if (check) {
+    ShenandoahHeap::heap()->assert_gc_workers(nworkers);
+  }
+}
+
+ShenandoahPushWorkerQueuesScope::~ShenandoahPushWorkerQueuesScope() {
+  assert(_workers->active_workers() == _n_workers,
+    "Active workers can not be changed within this scope");
+  // Restore old worker value
+  _queues->reserve(_old_workers);
+  _workers->update_active_workers(_old_workers);
+}
+
 AbstractGangWorker* ShenandoahWorkGang::install_worker(uint which) {
   AbstractGangWorker* worker = WorkGang::install_worker(which);
   ShenandoahThreadLocalData::create(worker);
diff --git a/src/hotspot/share/gc/shenandoah/shenandoahWorkGroup.hpp b/src/hotspot/share/gc/shenandoah/shenandoahWorkGroup.hpp
--- a/src/hotspot/share/gc/shenandoah/shenandoahWorkGroup.hpp
+++ b/src/hotspot/share/gc/shenandoah/shenandoahWorkGroup.hpp
@@ -27,6 +27,7 @@
 #include "gc/shared/workgroup.hpp"
 #include "memory/allocation.hpp"
 
+class ShenandoahObjToScanQueueSet;
 
 class ShenandoahWorkerScope : public StackObj {
 private:
@@ -49,6 +50,18 @@
   ~ShenandoahPushWorkerScope();
 };
 
+class ShenandoahPushWorkerQueuesScope : StackObj {
+private:
+  uint      _n_workers;
+  uint      _old_workers;
+  WorkGang* _workers;
+  ShenandoahObjToScanQueueSet* _queues;
+
+public:
+  ShenandoahPushWorkerQueuesScope(WorkGang* workers, ShenandoahObjToScanQueueSet* queues, uint nworkers, bool do_check = true);
+  ~ShenandoahPushWorkerQueuesScope();
+};
+
 class ShenandoahWorkGang : public WorkGang {
 private:
   bool     _initialize_gclab;