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diff --git a/src/base/spinlock.h b/src/base/spinlock.h
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+/* Copyright (c) 2006, Google Inc.
+ * All rights reserved.
+ * 
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are
+ * met:
+ * 
+ *     * Redistributions of source code must retain the above copyright
+ * notice, this list of conditions and the following disclaimer.
+ *     * Redistributions in binary form must reproduce the above
+ * copyright notice, this list of conditions and the following disclaimer
+ * in the documentation and/or other materials provided with the
+ * distribution.
+ *     * Neither the name of Google Inc. nor the names of its
+ * contributors may be used to endorse or promote products derived from
+ * this software without specific prior written permission.
+ * 
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+ * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+ * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+ * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+ * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+ * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+ * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ *
+ * ---
+ * Author: Sanjay Ghemawat
+ */
+
+//
+// Fast spinlocks (at least on x86, a lock/unlock pair is approximately
+// half the cost of a Mutex because the unlock just does a store instead
+// of a compare-and-swap which is expensive).
+
+// Spinlock is async signal safe.
+// If used within a signal handler, all lock holders 
+// should block the signal even outside the signal handler.
+
+#ifndef BASE_SPINLOCK_H__
+#define BASE_SPINLOCK_H__
+
+#include "base/basictypes.h"
+#include "base/atomicops.h"
+
+class SpinLock {
+ public:
+  SpinLock() : lockword_(0) { }
+
+  // Special constructor for use with static SpinLock objects.  E.g.,
+  //
+  //    static SpinLock lock(SpinLock::LINKER_INITIALIZED);
+  //
+  // When intialized using this constructor, we depend on the fact
+  // that the linker has already initialized the memory appropriately.
+  // A SpinLock constructed like this can be freely used from global
+  // initializers without worrying about the order in which global
+  // initializers run.
+  enum StaticInitializer { LINKER_INITIALIZED };
+  explicit SpinLock(StaticInitializer x) {
+    // Does nothing; lockword_ is already initialized
+  }
+
+  inline void Lock() {
+    if (Acquire_CompareAndSwap(&lockword_, 0, 1) != 0) {
+      SlowLock();
+    }
+  }
+
+  inline void Unlock() {
+    Release_Store(&lockword_, 0);
+  }
+
+  // Report if we think the lock can be held by this thread.
+  // When the lock is truly held by the invoking thread
+  // we will always return true.
+  // Indended to be used as CHECK(lock.IsHeld());
+  inline bool IsHeld() const {
+    return lockword_ != 0;
+  }
+
+ private:
+  // Lock-state: 0 means unlocked, 1 means locked
+  volatile AtomicWord lockword_;
+
+  void SlowLock();
+
+  DISALLOW_EVIL_CONSTRUCTORS(SpinLock);
+};
+
+// Corresponding locker object that arranges to acquire a spinlock for
+// the duration of a C++ scope.
+class SpinLockHolder {
+ private:
+  SpinLock* lock_;
+ public:
+  inline explicit SpinLockHolder(SpinLock* l) : lock_(l) { l->Lock(); }
+  inline ~SpinLockHolder() { lock_->Unlock(); }
+};
+// Catch bug where variable name is omitted, e.g. SpinLockHolder (&lock);
+#define SpinLockHolder(x) COMPILE_ASSERT(0, spin_lock_decl_missing_var_name)
+
+
+#endif  // BASE_SPINLOCK_H__