8357976: GenShen crash in swap_card_tables: Should be clean

Reviewed-by: kdnilsen, cslucas, ysr, xpeng

Author: William Kemper

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

@@ -415,10 +415,6 @@ void ShenandoahConcurrentGC::entry_reset() {
                                 msg);
     op_reset();
   }
-
-  if (heap->mode()->is_generational()) {
-    heap->old_generation()->card_scan()->mark_read_table_as_clean();
-  }
 }
 
 void ShenandoahConcurrentGC::entry_scan_remembered_set() {
@@ -644,6 +640,10 @@ void ShenandoahConcurrentGC::op_reset() {
   } else {
     _generation->prepare_gc();
   }
+
+  if (heap->mode()->is_generational()) {
+    heap->old_generation()->card_scan()->mark_read_table_as_clean();
+  }
 }
 
 class ShenandoahInitMarkUpdateRegionStateClosure : public ShenandoahHeapRegionClosure {

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

@@ -136,9 +136,15 @@ void ShenandoahDegenGC::op_degenerated() {
       heap->set_unload_classes(_generation->heuristics()->can_unload_classes() &&
                                 (!heap->mode()->is_generational() || _generation->is_global()));
 
-      if (heap->mode()->is_generational() && _generation->is_young()) {
-        // Swap remembered sets for young
-        _generation->swap_card_tables();
+      if (heap->mode()->is_generational()) {
+        // Clean the read table before swapping it. The end goal here is to have a clean
+        // write table, and to have the read table updated with the previous write table.
+        heap->old_generation()->card_scan()->mark_read_table_as_clean();
+
+        if (_generation->is_young()) {
+          // Swap remembered sets for young
+          _generation->swap_card_tables();
+        }
       }
 
     case _degenerated_roots: