A per-heap RegionTree needs no lock.

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

Index: third_party/WebKit/Source/platform/heap/PageMemory.cpp
diff --git a/third_party/WebKit/Source/platform/heap/PageMemory.cpp b/third_party/WebKit/Source/platform/heap/PageMemory.cpp
index 967c764c89762aca8e4bd97c018c80f0d3c07304..faa6548c6f3f07c9efcda8522efbdb5daeb1673d 100644
--- a/third_party/WebKit/Source/platform/heap/PageMemory.cpp
+++ b/third_party/WebKit/Source/platform/heap/PageMemory.cpp
@@ -72,7 +72,6 @@ PageMemoryRegion* PageMemoryRegion::allocate(size_t size,
 }
 
 PageMemoryRegion* RegionTree::lookup(Address address) {
-  MutexLocker locker(m_mutex);
   RegionTreeNode* current = m_root;
   while (current) {
     Address base = current->m_region->base();
@@ -93,7 +92,6 @@ PageMemoryRegion* RegionTree::lookup(Address address) {
 void RegionTree::add(PageMemoryRegion* region) {
   ASSERT(region);
   RegionTreeNode* newTree = new RegionTreeNode(region);
-  MutexLocker locker(m_mutex);
   newTree->addTo(&m_root);
 }
 
@@ -108,11 +106,6 @@ void RegionTreeNode::addTo(RegionTreeNode** context) {
 }
 
 void RegionTree::remove(PageMemoryRegion* region) {
-  // Deletion of large objects (and thus their regions) can happen
-  // concurrently on sweeper threads.  Removal can also happen during thread
-  // shutdown, but that case is safe.  Regardless, we make all removals
-  // mutually exclusive.
-  MutexLocker locker(m_mutex);
   ASSERT(region);
   ASSERT(m_root);
   Address base = region->base();