8328235: GenShen: Robustify ShenandoahGCSession and fix missing use

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

@@ -928,10 +928,7 @@ void ShenandoahEvacUpdateCleanupOopStorageRootsClosure::do_oop(oop* p) {
   const oop obj = RawAccess<>::oop_load(p);
   if (!CompressedOops::is_null(obj)) {
     if (!_mark_context->is_marked(obj)) {
-      // YSR TODO: This needs to be thought through carefully; in many of these
-      // cases, it makes more sense to place the state in the closure rather than
-      // do the async/snapshot field split we are doing with _gc_generation() and
-      // _active_generation().
+      _heap->assert_generations_reconciled();
       if (_heap->is_in_active_generation(obj)) {
         // TODO: This worries me. Here we are asserting that an unmarked from-space object is 'correct'.
         // Normally, I would call this a bogus assert, but there seems to be a legitimate use-case for

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

@@ -105,6 +105,7 @@ void ShenandoahSTWMark::mark() {
 
   // Weak reference processing
   ShenandoahReferenceProcessor* rp = heap->gc_generation()->ref_processor();
+  heap->assert_generations_reconciled();
   assert(heap->gc_generation() == heap->active_generation(), "Should be identical at stw phases");
   rp->reset_thread_locals();
   rp->set_soft_reference_policy(heap->soft_ref_policy()->should_clear_all_soft_refs());
@@ -174,8 +175,7 @@ void ShenandoahSTWMark::finish_mark(uint worker_id) {
   ShenandoahPhaseTimings::Phase phase = _full_gc ? ShenandoahPhaseTimings::full_gc_mark : ShenandoahPhaseTimings::degen_gc_stw_mark;
   ShenandoahWorkerTimingsTracker timer(phase, ShenandoahPhaseTimings::ParallelMark, worker_id);
   ShenandoahReferenceProcessor* rp = ShenandoahHeap::heap()->gc_generation()->ref_processor();
-  assert(ShenandoahHeap::heap()->gc_generation() == ShenandoahHeap::heap()->active_generation(),
-         "Should be identical at stw phases");
+  ShenandoahHeap::heap()->assert_generations_reconciled();
   StringDedup::Requests requests;
 
   mark_loop(worker_id, &_terminator, rp,