Lock CrossThreadPersistentRegion until end of weak processing.

third_party/WebKit/Source/platform/heap/PersistentNode.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 PersistentNode_h
 #define PersistentNode_h
 
 #include "platform/PlatformExport.h"
 #include "platform/heap/ThreadState.h"
 #include "wtf/Allocator.h"
@@ skipping 162 matching lines @@
         MutexLocker lock(m_mutex);
         return m_persistentRegion->allocatePersistentNode(self, trace);
     }
 
     void freePersistentNode(PersistentNode* persistentNode)
     {
         MutexLocker lock(m_mutex);
         m_persistentRegion->freePersistentNode(persistentNode);
     }
 
+    class LockScope final {
+        STACK_ALLOCATED();
+    public:
+        LockScope(CrossThreadPersistentRegion& persistentRegion)
+            : m_persistentRegion(persistentRegion)
+        {
+            m_persistentRegion.lock();
+        }
+        ~LockScope()
+        {
+            m_persistentRegion.unlock();
+        }
+    private:
+        CrossThreadPersistentRegion& m_persistentRegion;
+    };
+
     void tracePersistentNodes(Visitor* visitor)
     {
-        MutexLocker lock(m_mutex);
+        // If this assert triggers, you're tracing without being in a LockScope.
+        ASSERT(m_mutex.locked());
         m_persistentRegion->tracePersistentNodes(visitor);
     }
 
     void prepareForThreadStateTermination(ThreadState*);
 
 private:
+    friend class LockScope;
+
+    void lock()
+    {
+        m_mutex.lock();
+    }
+
+    void unlock()
+    {
+        m_mutex.unlock();
+    }
+
     // We don't make CrossThreadPersistentRegion inherit from PersistentRegion
     // because we don't want to virtualize performance-sensitive methods
     // such as PersistentRegion::allocate/freePersistentNode.
     OwnPtr<PersistentRegion> m_persistentRegion;
     Mutex m_mutex;
 };
 
 } // namespace blink
 
 #endif