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diff --git a/Source/JavaScriptCore/runtime/JSLock.cpp b/Source/JavaScriptCore/runtime/JSLock.cpp
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+++ b/Source/JavaScriptCore/runtime/JSLock.cpp
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+/*
+ * Copyright (C) 2005, 2008 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 NU
+ * 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 
+ *
+ */
+
+#include "config.h"
+#include "JSLock.h"
+
+#include "Collector.h"
+#include "CallFrame.h"
+
+#if ENABLE(JSC_MULTIPLE_THREADS)
+#include <pthread.h>
+#endif
+
+namespace JSC {
+
+#if ENABLE(JSC_MULTIPLE_THREADS)
+
+// Acquire this mutex before accessing lock-related data.
+static pthread_mutex_t JSMutex = PTHREAD_MUTEX_INITIALIZER;
+
+// Thread-specific key that tells whether a thread holds the JSMutex, and how many times it was taken recursively.
+pthread_key_t JSLockCount;
+
+static void createJSLockCount()
+{
+    pthread_key_create(&JSLockCount, 0);
+}
+
+pthread_once_t createJSLockCountOnce = PTHREAD_ONCE_INIT;
+
+// Lock nesting count.
+intptr_t JSLock::lockCount()
+{
+    pthread_once(&createJSLockCountOnce, createJSLockCount);
+
+    return reinterpret_cast<intptr_t>(pthread_getspecific(JSLockCount));
+}
+
+static void setLockCount(intptr_t count)
+{
+    ASSERT(count >= 0);
+    pthread_setspecific(JSLockCount, reinterpret_cast<void*>(count));
+}
+
+JSLock::JSLock(ExecState* exec)
+    : m_lockBehavior(exec->globalData().isSharedInstance() ? LockForReal : SilenceAssertionsOnly)
+{
+    lock(m_lockBehavior);
+}
+
+JSLock::JSLock(JSGlobalData* globalData)
+    : m_lockBehavior(globalData->isSharedInstance() ? LockForReal : SilenceAssertionsOnly)
+{
+    lock(m_lockBehavior);
+}
+
+void JSLock::lock(JSLockBehavior lockBehavior)
+{
+#ifdef NDEBUG
+    // Locking "not for real" is a debug-only feature.
+    if (lockBehavior == SilenceAssertionsOnly)
+        return;
+#endif
+
+    pthread_once(&createJSLockCountOnce, createJSLockCount);
+
+    intptr_t currentLockCount = lockCount();
+    if (!currentLockCount && lockBehavior == LockForReal) {
+        int result;
+        result = pthread_mutex_lock(&JSMutex);
+        ASSERT(!result);
+    }
+    setLockCount(currentLockCount + 1);
+}
+
+void JSLock::unlock(JSLockBehavior lockBehavior)
+{
+    ASSERT(lockCount());
+
+#ifdef NDEBUG
+    // Locking "not for real" is a debug-only feature.
+    if (lockBehavior == SilenceAssertionsOnly)
+        return;
+#endif
+
+    intptr_t newLockCount = lockCount() - 1;
+    setLockCount(newLockCount);
+    if (!newLockCount && lockBehavior == LockForReal) {
+        int result;
+        result = pthread_mutex_unlock(&JSMutex);
+        ASSERT(!result);
+    }
+}
+
+void JSLock::lock(ExecState* exec)
+{
+    lock(exec->globalData().isSharedInstance() ? LockForReal : SilenceAssertionsOnly);
+}
+
+void JSLock::unlock(ExecState* exec)
+{
+    unlock(exec->globalData().isSharedInstance() ? LockForReal : SilenceAssertionsOnly);
+}
+
+bool JSLock::currentThreadIsHoldingLock()
+{
+    pthread_once(&createJSLockCountOnce, createJSLockCount);
+    return !!pthread_getspecific(JSLockCount);
+}
+
+// This is fairly nasty.  We allow multiple threads to run on the same
+// context, and we do not require any locking semantics in doing so -
+// clients of the API may simply use the context from multiple threads
+// concurently, and assume this will work.  In order to make this work,
+// We lock the context when a thread enters, and unlock it when it leaves.
+// However we do not only unlock when the thread returns from its
+// entry point (evaluate script or call function), we also unlock the
+// context if the thread leaves JSC by making a call out to an external
+// function through a callback.
+//
+// All threads using the context share the same JS stack (the RegisterFile).
+// Whenever a thread calls into JSC it starts using the RegisterFile from the
+// previous 'high water mark' - the maximum point the stack has ever grown to
+// (returned by RegisterFile::end()).  So if a first thread calls out to a
+// callback, and a second thread enters JSC, then also exits by calling out
+// to a callback, we can be left with stackframes from both threads in the
+// RegisterFile.  As such, a problem may occur should the first thread's
+// callback complete first, and attempt to return to JSC.  Were we to allow
+// this to happen, and were its stack to grow further, then it may potentially
+// write over the second thread's call frames.
+//
+// In avoid JS stack corruption we enforce a policy of only ever allowing two
+// threads to use a JS context concurrently, and only allowing the second of
+// these threads to execute until it has completed and fully returned from its
+// outermost call into JSC.  We enforce this policy using 'lockDropDepth'.  The
+// first time a thread exits it will call DropAllLocks - which will do as expected
+// and drop locks allowing another thread to enter.  Should another thread, or the
+// same thread again, enter JSC (through evaluate script or call function), and exit
+// again through a callback, then the locks will not be dropped when DropAllLocks
+// is called (since lockDropDepth is non-zero).  Since this thread is still holding
+// the locks, only it will re able to re-enter JSC (either be returning from the
+// callback, or by re-entering through another call to evaulate script or call
+// function).
+//
+// This policy is slightly more restricive than it needs to be for correctness -
+// we could validly allow futher entries into JSC from other threads, we only
+// need ensure that callbacks return in the reverse chronological order of the
+// order in which they were made - though implementing the less restrictive policy
+// would likely increase complexity and overhead.
+//
+static unsigned lockDropDepth = 0;
+
+JSLock::DropAllLocks::DropAllLocks(ExecState* exec)
+    : m_lockBehavior(exec->globalData().isSharedInstance() ? LockForReal : SilenceAssertionsOnly)
+{
+    pthread_once(&createJSLockCountOnce, createJSLockCount);
+
+    if (lockDropDepth++) {
+        m_lockCount = 0;
+        return;
+    }
+
+    m_lockCount = JSLock::lockCount();
+    for (intptr_t i = 0; i < m_lockCount; i++)
+        JSLock::unlock(m_lockBehavior);
+}
+
+JSLock::DropAllLocks::DropAllLocks(JSLockBehavior JSLockBehavior)
+    : m_lockBehavior(JSLockBehavior)
+{
+    pthread_once(&createJSLockCountOnce, createJSLockCount);
+
+    if (lockDropDepth++) {
+        m_lockCount = 0;
+        return;
+    }
+
+    // It is necessary to drop even "unreal" locks, because having a non-zero lock count
+    // will prevent a real lock from being taken.
+
+    m_lockCount = JSLock::lockCount();
+    for (intptr_t i = 0; i < m_lockCount; i++)
+        JSLock::unlock(m_lockBehavior);
+}
+
+JSLock::DropAllLocks::~DropAllLocks()
+{
+    for (intptr_t i = 0; i < m_lockCount; i++)
+        JSLock::lock(m_lockBehavior);
+
+    --lockDropDepth;
+}
+
+#else
+
+JSLock::JSLock(ExecState*)
+    : m_lockBehavior(SilenceAssertionsOnly)
+{
+}
+
+// If threading support is off, set the lock count to a constant value of 1 so ssertions
+// that the lock is held don't fail
+intptr_t JSLock::lockCount()
+{
+    return 1;
+}
+
+bool JSLock::currentThreadIsHoldingLock()
+{
+    return true;
+}
+
+void JSLock::lock(JSLockBehavior)
+{
+}
+
+void JSLock::unlock(JSLockBehavior)
+{
+}
+
+void JSLock::lock(ExecState*)
+{
+}
+
+void JSLock::unlock(ExecState*)
+{
+}
+
+JSLock::DropAllLocks::DropAllLocks(ExecState*)
+{
+}
+
+JSLock::DropAllLocks::DropAllLocks(JSLockBehavior)
+{
+}
+
+JSLock::DropAllLocks::~DropAllLocks()
+{
+}
+
+#endif // USE(MULTIPLE_THREADS)
+
+} // namespace JSC