Add base::LazyInstance

base/lazy_instance.cc

+// Copyright 2008 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.
+
+#include "base/lazy_instance.h"
+
+#include "base/atomicops.h"
+#include "base/basictypes.h"
+
+#if defined(OS_POSIX)
+#include <sched.h>
+#elif defined(OS_WIN)
+#include <windows.h>
+#endif
+
+namespace {
+
+void ThreadYield() {
+#if defined(OS_POSIX)
+  sched_yield();
+#elif defined(OS_WIN)
+  Sleep(0);
+#endif
+}
+
+}  // namespace
+
+namespace base {
+namespace internal {
+
+// TODO(joth): This function could be shared with Singleton, in place of its
+// WaitForInstance() call.
+bool NeedsLazyInstance(subtle::AtomicWord* state) {
+  // Try to create the instance, if we're the first, will go from 0 to
+  // kLazyInstanceStateCreating, otherwise we've already been beaten here.
+  // The memory access has no memory ordering as state 0 and
+  // kLazyInstanceStateCreating have no associated data (memory barriers are
+  // all about ordering of memory accesses to *associated* data).
+  if (subtle::NoBarrier_CompareAndSwap(state, 0,
+                                       kLazyInstanceStateCreating) == 0)
+    // Caller must create instance
+    return true;
+
+  // It's either in the process of being created, or already created. Spin.
+  // The load has acquire memory ordering as a thread which sees
+  // state_ == STATE_CREATED needs to acquire visibility over
+  // the associated data (buf_). Pairing Release_Store is in
+  // CompleteLazyInstance().
+  while (subtle::Acquire_Load(state) == kLazyInstanceStateCreating) {
+    ThreadYield();
+  }
+  // Someone else created the instance.
+  return false;
+}
+
+void CompleteLazyInstance(subtle::AtomicWord* state,
+                          subtle::AtomicWord new_instance,
+                          void* lazy_instance) {
+  // Instance is created, go from CREATING to CREATED.
+  // Releases visibility over private_buf_ to readers. Pairing Acquire_Load's
+  // are in NeedsInstance() and Pointer().
+  subtle::Release_Store(state, new_instance);
+}
+
+}  // namespace internal
+}  // namespace base