8251910: Shenandoah: Handshake threads between weak-roots and reset phases

Reviewed-by: rkennke

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

@@ -424,6 +424,12 @@ void ShenandoahControlThread::service_concurrent_normal_cycle(GCCause::Cause cau
 
     // Update references freed up collection set, kick the cleanup to reclaim the space.
     heap->entry_cleanup_complete();
+  } else {
+    // Concurrent weak/strong root flags are unset concurrently. We depend on updateref GC safepoints
+    // to ensure the changes are visible to all mutators before gc cycle is completed.
+    // In case of no evacuation, updateref GC safepoints are skipped. Therefore, we will need
+    // to perform thread handshake to ensure their consistences.
+    heap->entry_rendezvous_roots();
   }
 
   // Cycle is complete

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

@@ -1821,8 +1821,7 @@ void ShenandoahHeap::op_weak_roots() {
     // Perform handshake to flush out dead oops
     {
       ShenandoahTimingsTracker t(ShenandoahPhaseTimings::conc_weak_roots_rendezvous);
-      ShenandoahRendezvousClosure cl;
+      rendezvous_threads();
-      Handshake::execute(&cl);
     }
   }
 }
@@ -1842,6 +1841,15 @@ void ShenandoahHeap::op_strong_roots() {
   set_concurrent_strong_root_in_progress(false);
 }
 
+void ShenandoahHeap::op_rendezvous_roots() {
+  rendezvous_threads();
+}
+
+void ShenandoahHeap::rendezvous_threads() {
+  ShenandoahRendezvousClosure cl;
+  Handshake::execute(&cl);
+}
+
 class ShenandoahResetUpdateRegionStateClosure : public ShenandoahHeapRegionClosure {
 private:
   ShenandoahMarkingContext* const _ctx;
@@ -2900,6 +2908,16 @@ void ShenandoahHeap::entry_cleanup_early() {
   op_cleanup_early();
 }
 
+void ShenandoahHeap::entry_rendezvous_roots() {
+  static const char* msg = "Rendezvous roots";
+  ShenandoahConcurrentPhase gc_phase(msg, ShenandoahPhaseTimings::conc_rendezvous_roots);
+  EventMark em("%s", msg);
+
+  // This phase does not use workers, no need for setup
+  try_inject_alloc_failure();
+  op_rendezvous_roots();
+}
+
 void ShenandoahHeap::entry_cleanup_complete() {
   static const char* msg = "Concurrent cleanup";
   ShenandoahConcurrentPhase gc_phase(msg, ShenandoahPhaseTimings::conc_cleanup_complete, true /* log_heap_usage */);

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

@@ -386,6 +386,7 @@ public:
   void entry_class_unloading();
   void entry_strong_roots();
   void entry_cleanup_early();
+  void entry_rendezvous_roots();
   void entry_evac();
   void entry_updaterefs();
   void entry_cleanup_complete();
@@ -409,12 +410,15 @@ private:
   void op_class_unloading();
   void op_strong_roots();
   void op_cleanup_early();
+  void op_rendezvous_roots();
   void op_conc_evac();
   void op_stw_evac();
   void op_updaterefs();
   void op_cleanup_complete();
   void op_uncommit(double shrink_before);
 
+  void rendezvous_threads();
+
   // Messages for GC trace events, they have to be immortal for
   // passing around the logging/tracing systems
   const char* init_mark_event_message() const;

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

@@ -99,6 +99,7 @@ class outputStream;
   f(conc_class_unload_purge_cldg,                   "    CLDG")                        \
   f(conc_class_unload_purge_ec,                     "    Exception Caches")            \
   f(conc_strong_roots,                              "Concurrent Strong Roots")         \
+  f(conc_rendezvous_roots,                          "Rendezvous")                      \
   SHENANDOAH_PAR_PHASE_DO(conc_strong_roots_,       "  CSR: ", f)                      \
   f(conc_evac,                                      "Concurrent Evacuation")           \
                                                                                        \