8329789: GenShen: Over assertive assert when scanning remembered set

Backport-of: 69281629a4a537b988f5dfd3a93733b3ddccd109

Author: William Kemper

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

@@ -772,7 +772,11 @@ void ShenandoahScanRemembered<RememberedSet>::process_clusters(size_t first_clus
     } else {
       // ==== BEGIN CLEAN card range processing ====
 
-      assert(ctbm[cur_index] == CardTable::clean_card_val(), "Error");
+      // If we are using the write table (during update refs, e.g.), a mutator may dirty
+      // a card at any time. This is fine for the algorithm below because it is only
+      // counting contiguous runs of clean cards (and only for non-product builds).
+      assert(use_write_table || ctbm[cur_index] == CardTable::clean_card_val(), "Error");
+
       // walk back over contiguous clean cards
       size_t i = 0;
       while (--cur_index >= (ssize_t)start_card_index && ctbm[cur_index] == CardTable::clean_card_val()) {