8307236: Rendezvous GC threads under STS for monitor deflation

Reviewed-by: eosterlund, shade

Author: rkennke

src/hotspot/share/runtime/monitorDeflationThread.hpp

@@ -41,6 +41,8 @@ class MonitorDeflationThread : public JavaThread {
 
   // Hide this thread from external view.
   bool is_hidden_from_external_view() const { return true; }
+
+  bool is_monitor_deflation_thread() const { return true; }
 };
 
 #endif // SHARE_RUNTIME_MONITORDEFLATIONTHREAD_HPP

src/hotspot/share/runtime/synchronizer.cpp

@@ -24,6 +24,7 @@
 
 #include "precompiled.hpp"
 #include "classfile/vmSymbols.hpp"
+#include "gc/shared/suspendibleThreadSet.hpp"
 #include "jfr/jfrEvents.hpp"
 #include "logging/log.hpp"
 #include "logging/logStream.hpp"
@@ -1473,6 +1474,16 @@ class HandshakeForDeflation : public HandshakeClosure {
   }
 };
 
+class VM_RendezvousGCThreads : public VM_Operation {
+public:
+  bool evaluate_at_safepoint() const override { return false; }
+  VMOp_Type type() const override { return VMOp_RendezvousGCThreads; }
+  void doit() override {
+    SuspendibleThreadSet::synchronize();
+    SuspendibleThreadSet::desynchronize();
+  };
+};
+
 // This function is called by the MonitorDeflationThread to deflate
 // ObjectMonitors. It is also called via do_final_audit_and_print_stats()
 // and VM_ThreadDump::doit() by the VMThread.
@@ -1514,7 +1525,7 @@ size_t ObjectSynchronizer::deflate_idle_monitors(ObjectMonitorsHashtable* table)
     ResourceMark rm;
     GrowableArray<ObjectMonitor*> delete_list((int)deflated_count);
     unlinked_count = _in_use_list.unlink_deflated(current, ls, &timer, &delete_list);
-    if (current->is_Java_thread()) {
+    if (current->is_monitor_deflation_thread()) {
       if (ls != nullptr) {
         timer.stop();
         ls->print_cr("before handshaking: unlinked_count=" SIZE_FORMAT
@@ -1528,13 +1539,22 @@ size_t ObjectSynchronizer::deflate_idle_monitors(ObjectMonitorsHashtable* table)
       // ObjectMonitors that were deflated in this cycle.
       HandshakeForDeflation hfd_hc;
       Handshake::execute(&hfd_hc);
+      // Also, we sync and desync GC threads around the handshake, so that they can
+      // safely read the mark-word and look-through to the object-monitor, without
+      // being afraid that the object-monitor is going away.
+      VM_RendezvousGCThreads sync_gc;
+      VMThread::execute(&sync_gc);
 
       if (ls != nullptr) {
         ls->print_cr("after handshaking: in_use_list stats: ceiling="
                      SIZE_FORMAT ", count=" SIZE_FORMAT ", max=" SIZE_FORMAT,
                      in_use_list_ceiling(), _in_use_list.count(), _in_use_list.max());
         timer.start();
       }
+    } else {
+      // This is not a monitor deflation thread.
+      // No handshake or rendezvous is needed when we are already at safepoint.
+      assert_at_safepoint();
     }
 
     // After the handshake, safely free the ObjectMonitors that were

src/hotspot/share/runtime/thread.hpp

@@ -319,6 +319,7 @@ class Thread: public ThreadShadow {
   virtual bool is_Named_thread() const               { return false; }
   virtual bool is_Worker_thread() const              { return false; }
   virtual bool is_JfrSampler_thread() const          { return false; }
+  virtual bool is_monitor_deflation_thread() const   { return false; }
 
   // Can this thread make Java upcalls
   virtual bool can_call_java() const                 { return false; }

src/hotspot/share/runtime/vmOperation.hpp

@@ -106,7 +106,8 @@
   template(GTestExecuteAtSafepoint)               \
   template(GTestStopSafepoint)                    \
   template(JFROldObject)                          \
-  template(JvmtiPostObjectFree)
+  template(JvmtiPostObjectFree)                   \
+  template(RendezvousGCThreads)
 
 class Thread;
 class outputStream;