8255070: Shenandoah: Use single thread for concurrent CLD liveness test

Reviewed-by: rkennke

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

@@ -1941,7 +1941,7 @@ private:
   ShenandoahVMWeakRoots<true /*concurrent*/> _vm_roots;
 
   // Roots related to concurrent class unloading
-  ShenandoahClassLoaderDataRoots<true /* concurrent */, false /* single thread*/>
+  ShenandoahClassLoaderDataRoots<true /* concurrent */, true /* single thread*/>
                                              _cld_roots;
   ShenandoahConcurrentNMethodIterator        _nmethod_itr;
   ShenandoahConcurrentStringDedupRoots       _dedup_roots;

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

@@ -149,6 +149,8 @@ private:
   ShenandoahPhaseTimings::Phase _phase;
 
   static uint worker_count(uint n_workers) {
+    if (SINGLE_THREADED) return 1u;
+
     // Limit concurrency a bit, otherwise it wastes resources when workers are tripping
     // over each other. This also leaves free workers to process other parts of the root
     // set, while admitted workers are busy with doing the CLDG walk.
@@ -161,6 +163,10 @@ public:
 
   void always_strong_cld_do(CLDClosure* clds, uint worker_id);
   void cld_do(CLDClosure* clds, uint worker_id);
+
+private:
+  typedef void (*CldDo)(CLDClosure*);
+  void cld_do_impl(CldDo f, CLDClosure* clds, uint worker_id);
 };
 
 class ShenandoahRootProcessor : public StackObj {

src/hotspot/share/gc/shenandoah/shenandoahRootProcessor.inline.hpp

@@ -35,6 +35,7 @@
 #include "gc/shenandoah/shenandoahUtils.hpp"
 #include "gc/shenandoah/heuristics/shenandoahHeuristics.hpp"
 #include "memory/resourceArea.hpp"
+#include "runtime/mutexLocker.hpp"
 #include "runtime/safepoint.hpp"
 
 template <bool CONCURRENT>
@@ -81,43 +82,49 @@ ShenandoahClassLoaderDataRoots<CONCURRENT, SINGLE_THREADED>::ShenandoahClassLoad
   if (!SINGLE_THREADED) {
     ClassLoaderDataGraph::clear_claimed_marks();
   }
-  if (CONCURRENT) {
+  if (CONCURRENT && !SINGLE_THREADED) {
     ClassLoaderDataGraph_lock->lock();
   }
+
+  // Non-concurrent mode only runs at safepoints by VM thread
+  assert(CONCURRENT || SafepointSynchronize::is_at_safepoint(), "Must be at a safepoint");
+  assert(CONCURRENT || Thread::current()->is_VM_thread(), "Can only be done by VM thread");
 }
 
 template <bool CONCURRENT, bool SINGLE_THREADED>
 ShenandoahClassLoaderDataRoots<CONCURRENT, SINGLE_THREADED>::~ShenandoahClassLoaderDataRoots() {
-  if (CONCURRENT) {
+  if (CONCURRENT && !SINGLE_THREADED) {
     ClassLoaderDataGraph_lock->unlock();
   }
 }
 
+template <bool CONCURRENT, bool SINGLE_THREADED>
+void ShenandoahClassLoaderDataRoots<CONCURRENT, SINGLE_THREADED>::cld_do_impl(CldDo f, CLDClosure* clds, uint worker_id) {
+  if (CONCURRENT) {
+    if (_semaphore.try_acquire()) {
+      ShenandoahWorkerTimingsTracker timer(_phase, ShenandoahPhaseTimings::CLDGRoots, worker_id);
+      if (SINGLE_THREADED){
+        MutexLocker ml(ClassLoaderDataGraph_lock, Mutex::_no_safepoint_check_flag);
+        f(clds);
+      } else {
+        f(clds);
+      }
+      _semaphore.claim_all();
+    }
+  } else {
+    f(clds);
+  }
+}
+
 
 template <bool CONCURRENT, bool SINGLE_THREADED>
 void ShenandoahClassLoaderDataRoots<CONCURRENT, SINGLE_THREADED>::always_strong_cld_do(CLDClosure* clds, uint worker_id) {
-  if (SINGLE_THREADED) {
+  cld_do_impl(&ClassLoaderDataGraph::always_strong_cld_do, clds, worker_id);
-    assert(SafepointSynchronize::is_at_safepoint(), "Must be at a safepoint");
-    assert(Thread::current()->is_VM_thread(), "Single threaded CLDG iteration can only be done by VM thread");
-    ClassLoaderDataGraph::always_strong_cld_do(clds);
-  } else if (_semaphore.try_acquire()) {
-    ShenandoahWorkerTimingsTracker timer(_phase, ShenandoahPhaseTimings::CLDGRoots, worker_id);
-    ClassLoaderDataGraph::always_strong_cld_do(clds);
-    _semaphore.claim_all();
-  }
 }
 
 template <bool CONCURRENT, bool SINGLE_THREADED>
 void ShenandoahClassLoaderDataRoots<CONCURRENT, SINGLE_THREADED>::cld_do(CLDClosure* clds, uint worker_id) {
-  if (SINGLE_THREADED) {
+  cld_do_impl(&ClassLoaderDataGraph::cld_do, clds, worker_id);
-    assert(SafepointSynchronize::is_at_safepoint(), "Must be at a safepoint");
-    assert(Thread::current()->is_VM_thread(), "Single threaded CLDG iteration can only be done by VM thread");
-    ClassLoaderDataGraph::cld_do(clds);
-  } else if (_semaphore.try_acquire()) {
-    ShenandoahWorkerTimingsTracker timer(_phase, ShenandoahPhaseTimings::CLDGRoots, worker_id);
-    ClassLoaderDataGraph::cld_do(clds);
-    _semaphore.claim_all();
-  }
 }
 
 class ShenandoahParallelOopsDoThreadClosure : public ThreadClosure {