--- old/src/hotspot/share/gc/g1/g1HotCardCache.cpp	2019-10-22 14:06:41.581275329 +0200
+++ new/src/hotspot/share/gc/g1/g1HotCardCache.cpp	2019-10-22 14:06:41.241264657 +0200
@@ -32,7 +32,7 @@
 G1HotCardCache::G1HotCardCache(G1CollectedHeap *g1h):
   _g1h(g1h), _use_cache(false), _card_counts(g1h),
   _hot_cache(NULL), _hot_cache_size(0), _hot_cache_par_chunk_size(0),
-  _hot_cache_idx(0), _hot_cache_par_claimed_idx(0)
+  _hot_cache_idx(0), _hot_cache_par_claimed_idx(0), _cache_wrapped_around(false)
 {}
 
 void G1HotCardCache::initialize(G1RegionToSpaceMapper* card_counts_storage) {
@@ -48,6 +48,8 @@
     _hot_cache_par_chunk_size = ClaimChunkSize;
     _hot_cache_par_claimed_idx = 0;
 
+    _cache_wrapped_around = false;
+
     _card_counts.initialize(card_counts_storage);
   }
 }
@@ -69,6 +71,11 @@
   }
   // Otherwise, the card is hot.
   size_t index = Atomic::add(1u, &_hot_cache_idx) - 1;
+  // This does not need an atomic update. Racing threads may at most write the
+  // same value.
+  if (index == _hot_cache_size) {
+    _cache_wrapped_around = true;
+  }
   size_t masked_index = index & (_hot_cache_size - 1);
   CardValue* current_ptr = _hot_cache[masked_index];