A per-heap RegionTree needs no lock.

third_party/WebKit/Source/platform/heap/PageMemory.h

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef PageMemory_h
 #define PageMemory_h
 
 #include "platform/heap/HeapPage.h"
 #include "wtf/Allocator.h"
 #include "wtf/Assertions.h"
 #include "wtf/Compiler.h"
 #include "wtf/allocator/Partitions.h"
 
-#if OS(POSIX)
-#include <sys/mman.h>
-#include <unistd.h>
-#endif
-
 namespace blink {
 
 class RegionTree;
 class RegionTreeNode;
 
 class MemoryRegion {
   USING_FAST_MALLOC(MemoryRegion);
 
  public:
   MemoryRegion(Address base, size_t size) : m_base(base), m_size(size) {
@@ skipping 86 matching lines @@
   USING_FAST_MALLOC(RegionTree);
 
  public:
   RegionTree() : m_root(nullptr) {}
 
   void add(PageMemoryRegion*);
   void remove(PageMemoryRegion*);
   PageMemoryRegion* lookup(Address);
 
  private:
-  Mutex m_mutex;
   RegionTreeNode* m_root;
 };
 
 class RegionTreeNode {
   USING_FAST_MALLOC(RegionTreeNode);
 
  public:
   explicit RegionTreeNode(PageMemoryRegion* region)
       : m_region(region), m_left(nullptr), m_right(nullptr) {}
 
@@ skipping 69 matching lines @@
  private:
   PageMemory(PageMemoryRegion* reserved, const MemoryRegion& writable);
 
   PageMemoryRegion* m_reserved;
   MemoryRegion m_writable;
 };
 
 }  // namespace blink
 
 #endif