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diff --git a/Source/JavaScriptCore/runtime/JSLock.h b/Source/JavaScriptCore/runtime/JSLock.h
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+/*
+ * Copyright (C) 2005, 2008, 2009 Apple Inc. All rights reserved.
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Library General Public
+ * License as published by the Free Software Foundation; either
+ * version 2 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Library General Public License for more details.
+ *
+ * You should have received a copy of the GNU Library General Public License
+ * along with this library; see the file COPYING.LIB.  If not, write to
+ * the Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor,
+ * Boston, MA 02110-1301, USA.
+ *
+ */
+
+#ifndef JSLock_h
+#define JSLock_h
+
+#include <wtf/Assertions.h>
+#include <wtf/Noncopyable.h>
+
+namespace JSC {
+
+    // To make it safe to use JavaScript on multiple threads, it is
+    // important to lock before doing anything that allocates a
+    // JavaScript data structure or that interacts with shared state
+    // such as the protect count hash table. The simplest way to lock
+    // is to create a local JSLock object in the scope where the lock 
+    // must be held. The lock is recursive so nesting is ok. The JSLock 
+    // object also acts as a convenience short-hand for running important
+    // initialization routines.
+
+    // To avoid deadlock, sometimes it is necessary to temporarily
+    // release the lock. Since it is recursive you actually have to
+    // release all locks held by your thread. This is safe to do if
+    // you are executing code that doesn't require the lock, and you
+    // reacquire the right number of locks at the end. You can do this
+    // by constructing a locally scoped JSLock::DropAllLocks object. The 
+    // DropAllLocks object takes care to release the JSLock only if your
+    // thread acquired it to begin with.
+
+    // For contexts other than the single shared one, implicit locking is not done,
+    // but we still need to perform all the counting in order to keep debug
+    // assertions working, so that clients that use the shared context don't break.
+
+    class ExecState;
+    class JSGlobalData;
+
+    enum JSLockBehavior { SilenceAssertionsOnly, LockForReal };
+
+    class JSLock : public Noncopyable {
+    public:
+        JSLock(ExecState*);
+        JSLock(JSGlobalData*);
+
+        JSLock(JSLockBehavior lockBehavior)
+            : m_lockBehavior(lockBehavior)
+        {
+#ifdef NDEBUG
+            // Locking "not for real" is a debug-only feature.
+            if (lockBehavior == SilenceAssertionsOnly)
+                return;
+#endif
+            lock(lockBehavior);
+        }
+
+        ~JSLock()
+        { 
+#ifdef NDEBUG
+            // Locking "not for real" is a debug-only feature.
+            if (m_lockBehavior == SilenceAssertionsOnly)
+                return;
+#endif
+            unlock(m_lockBehavior); 
+        }
+        
+        static void lock(JSLockBehavior);
+        static void unlock(JSLockBehavior);
+        static void lock(ExecState*);
+        static void unlock(ExecState*);
+
+        static intptr_t lockCount();
+        static bool currentThreadIsHoldingLock();
+
+        JSLockBehavior m_lockBehavior;
+
+        class DropAllLocks : public Noncopyable {
+        public:
+            DropAllLocks(ExecState* exec);
+            DropAllLocks(JSLockBehavior);
+            ~DropAllLocks();
+            
+        private:
+            intptr_t m_lockCount;
+            JSLockBehavior m_lockBehavior;
+        };
+    };
+
+} // namespace
+
+#endif // JSLock_h