8352918: Shenandoah: Verifier does not deactivate barriers as intended

Reviewed-by: kdnilsen, shade, ysr

Author: William Kemper

src/hotspot/share/gc/shenandoah/shenandoahRootVerifier.cpp

@@ -46,13 +46,19 @@
 
 ShenandoahGCStateResetter::ShenandoahGCStateResetter() :
   _heap(ShenandoahHeap::heap()),
-  _gc_state(_heap->gc_state()) {
+  _saved_gc_state(_heap->gc_state()),
+  _saved_gc_state_changed(_heap->_gc_state_changed) {
+  // Clear state to deactivate barriers. Indicate that state has changed
+  // so that verifier threads will use this value, rather than thread local
+  // values (which we are _not_ changing here).
   _heap->_gc_state.clear();
+  _heap->_gc_state_changed = true;
 }
 
 ShenandoahGCStateResetter::~ShenandoahGCStateResetter() {
-  _heap->_gc_state.set(_gc_state);
-  assert(_heap->gc_state() == _gc_state, "Should be restored");
+  _heap->_gc_state.set(_saved_gc_state);
+  _heap->_gc_state_changed = _saved_gc_state_changed;
+  assert(_heap->gc_state() == _saved_gc_state, "Should be restored");
 }
 
 void ShenandoahRootVerifier::roots_do(OopIterateClosure* oops) {

src/hotspot/share/gc/shenandoah/shenandoahRootVerifier.hpp

@@ -32,7 +32,8 @@
 class ShenandoahGCStateResetter : public StackObj {
 private:
   ShenandoahHeap* const _heap;
-  const char _gc_state;
+  const char _saved_gc_state;
+  const bool _saved_gc_state_changed;
 
 public:
   ShenandoahGCStateResetter();