8320525: G1: G1UpdateRemSetTrackingBeforeRebuild::distribute_marked_bytes accesses partially unloaded klass

Reviewed-by: ayang, iwalulya

Author: Thomas Schatzl

src/hotspot/share/gc/g1/g1ConcurrentMark.cpp

@@ -1131,13 +1131,10 @@ class G1UpdateRemSetTrackingBeforeRebuildTask : public WorkerTask {
     // Distribute the given marked bytes across the humongous object starting
     // with hr and note end of marking for these regions.
     void distribute_marked_bytes(HeapRegion* hr, size_t marked_bytes) {
-      size_t const obj_size_in_words = cast_to_oop(hr->bottom())->size();
-
-      // "Distributing" zero words means that we only note end of marking for these
-      // regions.
-      assert(marked_bytes == 0 || obj_size_in_words * HeapWordSize == marked_bytes,
+      // Dead humongous objects (marked_bytes == 0) may have already been unloaded.
+      assert(marked_bytes == 0 || cast_to_oop(hr->bottom())->size() * HeapWordSize == marked_bytes,
              "Marked bytes should either be 0 or the same as humongous object (%zu) but is %zu",
-             obj_size_in_words * HeapWordSize, marked_bytes);
+             cast_to_oop(hr->bottom())->size() * HeapWordSize, marked_bytes);
 
       auto distribute_bytes = [&] (HeapRegion* r) {
         size_t const bytes_to_add = MIN2(HeapRegion::GrainBytes, marked_bytes);
@@ -1148,10 +1145,6 @@ class G1UpdateRemSetTrackingBeforeRebuildTask : public WorkerTask {
         marked_bytes -= bytes_to_add;
       };
       _g1h->humongous_obj_regions_iterate(hr, distribute_bytes);
-
-      assert(marked_bytes == 0,
-             "%zu bytes left after distributing space across %zu regions",
-             marked_bytes, G1CollectedHeap::humongous_obj_size_in_regions(obj_size_in_words));
     }
 
     void update_marked_bytes(HeapRegion* hr) {