1 files changed, 69 insertions, 0 deletions
diff --git a/security/sandbox/chromium/base/atomic_ref_count.h b/security/sandbox/chromium/base/atomic_ref_count.h
new file mode 100644
index 0000000000..5e48c82380
--- /dev/null
+++ b/security/sandbox/chromium/base/atomic_ref_count.h
@@ -0,0 +1,69 @@
+// Copyright (c) 2011 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.
+
+// This is a low level implementation of atomic semantics for reference
+// counting.  Please use base/memory/ref_counted.h directly instead.
+
+#ifndef BASE_ATOMIC_REF_COUNT_H_
+#define BASE_ATOMIC_REF_COUNT_H_
+
+#include <atomic>
+
+namespace base {
+
+class AtomicRefCount {
+ public:
+  constexpr AtomicRefCount() : ref_count_(0) {}
+  explicit constexpr AtomicRefCount(int initial_value)
+      : ref_count_(initial_value) {}
+
+  // Increment a reference count.
+  // Returns the previous value of the count.
+  int Increment() { return Increment(1); }
+
+  // Increment a reference count by "increment", which must exceed 0.
+  // Returns the previous value of the count.
+  int Increment(int increment) {
+    return ref_count_.fetch_add(increment, std::memory_order_relaxed);
+  }
+
+  // Decrement a reference count, and return whether the result is non-zero.
+  // Insert barriers to ensure that state written before the reference count
+  // became zero will be visible to a thread that has just made the count zero.
+  bool Decrement() {
+    // TODO(jbroman): Technically this doesn't need to be an acquire operation
+    // unless the result is 1 (i.e., the ref count did indeed reach zero).
+    // However, there are toolchain issues that make that not work as well at
+    // present (notably TSAN doesn't like it).
+    return ref_count_.fetch_sub(1, std::memory_order_acq_rel) != 1;
+  }
+
+  // Return whether the reference count is one.  If the reference count is used
+  // in the conventional way, a refrerence count of 1 implies that the current
+  // thread owns the reference and no other thread shares it.  This call
+  // performs the test for a reference count of one, and performs the memory
+  // barrier needed for the owning thread to act on the object, knowing that it
+  // has exclusive access to the object.
+  bool IsOne() const { return ref_count_.load(std::memory_order_acquire) == 1; }
+
+  // Return whether the reference count is zero.  With conventional object
+  // referencing counting, the object will be destroyed, so the reference count
+  // should never be zero.  Hence this is generally used for a debug check.
+  bool IsZero() const {
+    return ref_count_.load(std::memory_order_acquire) == 0;
+  }
+
+  // Returns the current reference count (with no barriers). This is subtle, and
+  // should be used only for debugging.
+  int SubtleRefCountForDebug() const {
+    return ref_count_.load(std::memory_order_relaxed);
+  }
+
+ private:
+  std::atomic_int ref_count_;
+};
+
+}  // namespace base
+
+#endif  // BASE_ATOMIC_REF_COUNT_H_