diff --git a/gc.c b/gc.c
index 82962a9a..5a1c4b06 100644
--- a/gc.c
+++ b/gc.c
@@ -480,6 +480,10 @@ static const int  gc_color_blue = 3;
 static int gc_status_col;
 static int gc_stage;
 
+// Does not need sync, only used by collector thread
+static void **mark_stack;
+static int mark_stack_len;
+
 // GC functions called by the Mutator threads
 
 void gc_mut_update()
diff --git a/include/cyclone/types.h b/include/cyclone/types.h
index d0fb9fa1..e8a03c16 100644
--- a/include/cyclone/types.h
+++ b/include/cyclone/types.h
@@ -25,8 +25,13 @@ typedef struct gc_thread_data_t gc_thread_data;
 struct gc_thread_data_t {
   void **moveBuf; /* list of objects moved to heap during GC */
   int moveBufLen;
-  int gc_alloc_color; // For tri-color marking
+  // Data needed for tri-color marking 
+  int gc_alloc_color;
   int gc_mut_status;
+  int last_write;
+  int last_read;
+  void **mark_buffer;
+  int mark_buffer_len;
 };
 
 /* GC data structures */