6914402: G1: assert(!is_young_card(cached_ptr),"shouldn't get a card in young region")

Summary: Invalid assert. Filter cards evicted from the card count cache instead.
Reviewed-by: apetrusenko, tonyp

src/share/vm/gc_implementation/g1/concurrentG1Refine.cpp

@@ -300,7 +300,23 @@ jbyte* ConcurrentG1Refine::cache_insert(jbyte* card_ptr, bool* defer) {
   int count;
   jbyte* cached_ptr = add_card_count(card_ptr, &count, defer);
   assert(cached_ptr != NULL, "bad cached card ptr");
-  assert(!is_young_card(cached_ptr), "shouldn't get a card in young region");
+
+  if (is_young_card(cached_ptr)) {
+    // The region containing cached_ptr has been freed during a clean up
+    // pause, reallocated, and tagged as young.
+    assert(cached_ptr != card_ptr, "shouldn't be");
+
+    // We've just inserted a new old-gen card pointer into the card count
+    // cache and evicted the previous contents of that count slot.
+    // The evicted card pointer has been determined to be in a young region
+    // and so cannot be the newly inserted card pointer (that will be
+    // in an old region).
+    // The count for newly inserted card will be set to zero during the
+    // insertion, so we don't want to defer the cleaning of the newly
+    // inserted card pointer.
+    assert(*defer == false, "deferring non-hot card");
+    return NULL;
+  }
 
   // The card pointer we obtained from card count cache is not hot
   // so do not store it in the cache; return it for immediate