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 179 matching lines @@
         // the CrossThreadPersistent<> also clearing the persistent and freeing the PersistentNode.
         //
         // The lock ensures the updating is ordered, but by the time lock has been acquired the
         // PersistentNode reference may have been cleared out already; check for this.
         if (!persistentNode)
             return;
         m_persistentRegion->freePersistentNode(persistentNode);
         releaseStore(reinterpret_cast<void* volatile*>(&persistentNode), nullptr);
     }
 
+    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.
+#if ENABLE(ASSERT)
+        DCHECK(m_mutex.locked());
+#endif
         m_persistentRegion->tracePersistentNodes(visitor, CrossThreadPersistentRegion::shouldTracePersistentNode);
     }
 
     void prepareForThreadStateTermination(ThreadState*);
 
     static bool shouldTracePersistentNode(Visitor*, PersistentNode*);
 
 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;
 
     // Recursive as prepareForThreadStateTermination() clears a PersistentNode's
     // associated Persistent<> -- it in turn freeing the PersistentNode. And both
     // CrossThreadPersistentRegion operations need a lock on the region before
     // mutating.
     RecursiveMutex m_mutex;
 };
 
 } // namespace blink
 
 #endif