diff --git a/src/heap.h b/src/heap.h
index a6fb4d4..e205f7a 100644
--- a/src/heap.h
+++ b/src/heap.h
@@ -43,6 +43,7 @@
 class Thread;
 
 class Heap {
+  friend class ScopedHeapLock;
  public:
   static const size_t kInitialSize = 2 * MB;
 
@@ -123,8 +124,6 @@
   void WaitForConcurrentGcToComplete();
 
   pid_t GetLockOwner(); // For SignalCatcher.
-  void Lock();
-  void Unlock();
   void AssertLockHeld() {
     lock_->AssertHeld();
   }
@@ -242,6 +241,9 @@
   Object* AllocateLocked(size_t num_bytes);
   Object* AllocateLocked(AllocSpace* space, size_t num_bytes);
 
+  void Lock();
+  void Unlock();
+
   // Pushes a list of cleared references out to the managed heap.
   void EnqueueClearedReferences(Object** cleared_references);
 
@@ -289,7 +291,7 @@
   bool card_marking_disabled_;
 
   // True while the garbage collector is running.
-  bool is_gc_running_;
+  volatile bool is_gc_running_;
 
   // Bytes until concurrent GC
   size_t concurrent_start_bytes_;