8243238: Shenandoah: explicit GC request should wait for a complete GC cycle

Reviewed-by: rkennke

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

@@ -48,6 +48,7 @@ ShenandoahControlThread::ShenandoahControlThread() :
   _degen_point(ShenandoahHeap::_degenerated_outside_cycle),
   _allocs_seen(0) {
 
+  reset_gc_id();
   create_and_start(ShenandoahCriticalControlThreadPriority ? CriticalPriority : NearMaxPriority);
   _periodic_task.enroll();
   _periodic_satb_flush_task.enroll();
@@ -173,6 +174,9 @@ void ShenandoahControlThread::run_service() {
     assert (!gc_requested || cause != GCCause::_last_gc_cause, "GC cause should be set");
 
     if (gc_requested) {
+      // GC is starting, bump the internal ID
+      update_gc_id();
+
       heap->reset_bytes_allocated_since_gc_start();
 
       // Use default constructor to snapshot the Metaspace state before GC.
@@ -474,10 +478,20 @@ void ShenandoahControlThread::request_gc(GCCause::Cause cause) {
 }
 
 void ShenandoahControlThread::handle_requested_gc(GCCause::Cause cause) {
-  _requested_gc_cause = cause;
-  _gc_requested.set();
+  // Make sure we have at least one complete GC cycle before unblocking
+  // from the explicit GC request.
+  //
+  // This is especially important for weak references cleanup and/or native
+  // resources (e.g. DirectByteBuffers) machinery: when explicit GC request
+  // comes very late in the already running cycle, it would miss lots of new
+  // opportunities for cleanup that were made available before the caller
+  // requested the GC.
+  size_t required_gc_id = get_gc_id() + 1;
+
   MonitorLocker ml(&_gc_waiters_lock);
-  while (_gc_requested.is_set()) {
+  while (get_gc_id() < required_gc_id) {
+    _gc_requested.set();
+    _requested_gc_cause = cause;
     ml.wait();
   }
 }
@@ -573,6 +587,18 @@ void ShenandoahControlThread::set_forced_counters_update(bool value) {
   _force_counters_update.set_cond(value);
 }
 
+void ShenandoahControlThread::reset_gc_id() {
+  Atomic::store(&_gc_id, (size_t)0);
+}
+
+void ShenandoahControlThread::update_gc_id() {
+  Atomic::inc(&_gc_id);
+}
+
+size_t ShenandoahControlThread::get_gc_id() {
+  return Atomic::load(&_gc_id);
+}
+
 void ShenandoahControlThread::print() const {
   print_on(tty);
 }

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

@@ -88,6 +88,8 @@ private:
   shenandoah_padding(0);
   volatile size_t _allocs_seen;
   shenandoah_padding(1);
+  volatile size_t _gc_id;
+  shenandoah_padding(2);
 
   bool check_cancellation_or_degen(ShenandoahHeap::ShenandoahDegenPoint point);
   void service_concurrent_normal_cycle(GCCause::Cause cause);
@@ -99,6 +101,10 @@ private:
   void notify_alloc_failure_waiters();
   bool is_alloc_failure_gc();
 
+  void reset_gc_id();
+  void update_gc_id();
+  size_t get_gc_id();
+
   void notify_gc_waiters();
 
   // Handle GC request.