8322503: Shenandoah: Clarify gc state usage

Reviewed-by: shade
Backport-of: 51238c4bdbce48f6b719f7dcfe5b7df8b8c6d85b

Author: William Kemper

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

@@ -1702,7 +1702,7 @@ void ShenandoahHeap::prepare_update_heap_references(bool concurrent) {
   _update_refs_iterator.reset();
 }
 
-void ShenandoahHeap::set_gc_state_all_threads() {
+void ShenandoahHeap::propagate_gc_state_to_java_threads() {
   assert(ShenandoahSafepoint::is_at_shenandoah_safepoint(), "Must be at Shenandoah safepoint");
   if (_gc_state_changed) {
     _gc_state_changed = false;
@@ -1713,21 +1713,21 @@ void ShenandoahHeap::set_gc_state_all_threads() {
   }
 }
 
-void ShenandoahHeap::set_gc_state_mask(uint mask, bool value) {
+void ShenandoahHeap::set_gc_state(uint mask, bool value) {
   assert(ShenandoahSafepoint::is_at_shenandoah_safepoint(), "Must be at Shenandoah safepoint");
   _gc_state.set_cond(mask, value);
   _gc_state_changed = true;
 }
 
 void ShenandoahHeap::set_concurrent_mark_in_progress(bool in_progress) {
   assert(!has_forwarded_objects(), "Not expected before/after mark phase");
-  set_gc_state_mask(MARKING, in_progress);
+  set_gc_state(MARKING, in_progress);
   ShenandoahBarrierSet::satb_mark_queue_set().set_active_all_threads(in_progress, !in_progress);
 }
 
 void ShenandoahHeap::set_evacuation_in_progress(bool in_progress) {
   assert(ShenandoahSafepoint::is_at_shenandoah_safepoint(), "Only call this at safepoint");
-  set_gc_state_mask(EVACUATION, in_progress);
+  set_gc_state(EVACUATION, in_progress);
 }
 
 void ShenandoahHeap::set_concurrent_strong_root_in_progress(bool in_progress) {
@@ -1739,7 +1739,7 @@ void ShenandoahHeap::set_concurrent_strong_root_in_progress(bool in_progress) {
 }
 
 void ShenandoahHeap::set_concurrent_weak_root_in_progress(bool cond) {
-  set_gc_state_mask(WEAK_ROOTS, cond);
+  set_gc_state(WEAK_ROOTS, cond);
 }
 
 GCTracer* ShenandoahHeap::tracer() {
@@ -1857,7 +1857,7 @@ void ShenandoahHeap::parallel_cleaning(bool full_gc) {
 }
 
 void ShenandoahHeap::set_has_forwarded_objects(bool cond) {
-  set_gc_state_mask(HAS_FORWARDED, cond);
+  set_gc_state(HAS_FORWARDED, cond);
 }
 
 void ShenandoahHeap::set_unload_classes(bool uc) {
@@ -1900,7 +1900,7 @@ void ShenandoahHeap::set_full_gc_move_in_progress(bool in_progress) {
 }
 
 void ShenandoahHeap::set_update_refs_in_progress(bool in_progress) {
-  set_gc_state_mask(UPDATEREFS, in_progress);
+  set_gc_state(UPDATEREFS, in_progress);
 }
 
 void ShenandoahHeap::register_nmethod(nmethod* nm) {

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

@@ -289,12 +289,19 @@ class ShenandoahHeap : public CollectedHeap {
   ShenandoahSharedFlag   _progress_last_gc;
   ShenandoahSharedFlag   _concurrent_strong_root_in_progress;
 
-  void set_gc_state_mask(uint mask, bool value);
+  // This updates the singlular, global gc state. This must happen on a safepoint.
+  void set_gc_state(uint mask, bool value);
 
 public:
   char gc_state() const;
-  void set_gc_state_all_threads();
-  bool has_gc_state_changed() { return _gc_state_changed; }
+
+  // This copies the global gc state into a thread local variable for java threads.
+  // It is primarily intended to support quick access at barriers.
+  void propagate_gc_state_to_java_threads();
+
+  // This is public to support assertions that the state hasn't been changed off of
+  // a safepoint and that any changes were propagated to java threads after the safepoint.
+  bool has_gc_state_changed() const { return _gc_state_changed; }
 
   void set_concurrent_mark_in_progress(bool in_progress);
   void set_evacuation_in_progress(bool in_progress);

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

@@ -100,6 +100,7 @@ class ShenandoahThreadLocalData {
   }
 
   static char gc_state(Thread* thread) {
+    assert(thread->is_Java_thread(), "GC state is only synchronized to java threads");
     return data(thread)->_gc_state;
   }
 

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

@@ -62,41 +62,41 @@ void VM_ShenandoahReferenceOperation::doit_epilogue() {
 void VM_ShenandoahInitMark::doit() {
   ShenandoahGCPauseMark mark(_gc_id, "Init Mark", SvcGCMarker::CONCURRENT);
   _gc->entry_init_mark();
-  ShenandoahHeap::heap()->set_gc_state_all_threads();
+  ShenandoahHeap::heap()->propagate_gc_state_to_java_threads();
 }
 
 void VM_ShenandoahFinalMarkStartEvac::doit() {
   ShenandoahGCPauseMark mark(_gc_id, "Final Mark", SvcGCMarker::CONCURRENT);
   _gc->entry_final_mark();
-  ShenandoahHeap::heap()->set_gc_state_all_threads();
+  ShenandoahHeap::heap()->propagate_gc_state_to_java_threads();
 }
 
 void VM_ShenandoahFullGC::doit() {
   ShenandoahGCPauseMark mark(_gc_id, "Full GC", SvcGCMarker::FULL);
   _full_gc->entry_full(_gc_cause);
-  ShenandoahHeap::heap()->set_gc_state_all_threads();
+  ShenandoahHeap::heap()->propagate_gc_state_to_java_threads();
 }
 
 void VM_ShenandoahDegeneratedGC::doit() {
   ShenandoahGCPauseMark mark(_gc_id, "Degenerated GC", SvcGCMarker::CONCURRENT);
   _gc->entry_degenerated();
-  ShenandoahHeap::heap()->set_gc_state_all_threads();
+  ShenandoahHeap::heap()->propagate_gc_state_to_java_threads();
 }
 
 void VM_ShenandoahInitUpdateRefs::doit() {
   ShenandoahGCPauseMark mark(_gc_id, "Init Update Refs", SvcGCMarker::CONCURRENT);
   _gc->entry_init_updaterefs();
-  ShenandoahHeap::heap()->set_gc_state_all_threads();
+  ShenandoahHeap::heap()->propagate_gc_state_to_java_threads();
 }
 
 void VM_ShenandoahFinalUpdateRefs::doit() {
   ShenandoahGCPauseMark mark(_gc_id, "Final Update Refs", SvcGCMarker::CONCURRENT);
   _gc->entry_final_updaterefs();
-  ShenandoahHeap::heap()->set_gc_state_all_threads();
+  ShenandoahHeap::heap()->propagate_gc_state_to_java_threads();
 }
 
 void VM_ShenandoahFinalRoots::doit() {
   ShenandoahGCPauseMark mark(_gc_id, "Final Roots", SvcGCMarker::CONCURRENT);
   _gc->entry_final_roots();
-  ShenandoahHeap::heap()->set_gc_state_all_threads();
+  ShenandoahHeap::heap()->propagate_gc_state_to_java_threads();
 }

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

@@ -619,7 +619,7 @@ void ShenandoahVerifier::verify_at_safepoint(const char *label,
   guarantee(ShenandoahSafepoint::is_at_shenandoah_safepoint(), "only when nothing else happens");
   guarantee(ShenandoahVerify, "only when enabled, and bitmap is initialized in ShenandoahHeap::initialize");
 
-  ShenandoahHeap::heap()->set_gc_state_all_threads();
+  ShenandoahHeap::heap()->propagate_gc_state_to_java_threads();
 
   // Avoid side-effect of changing workers' active thread count, but bypass concurrent/parallel protocol check
   ShenandoahPushWorkerScope verify_worker_scope(_heap->workers(), _heap->max_workers(), false /*bypass check*/);