Repo created

TMessagesProj/jni/voip/webrtc/base/lazy_instance_helpers.cc

@@ -0,0 +1,64 @@
+// Copyright 2018 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_helpers.h"
+
+#include "base/at_exit.h"
+#include "base/atomicops.h"
+#include "base/threading/platform_thread.h"
+
+namespace base {
+namespace internal {
+
+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().
+  if (subtle::Acquire_Load(state) == kLazyInstanceStateCreating) {
+    const base::TimeTicks start = base::TimeTicks::Now();
+    do {
+      const base::TimeDelta elapsed = base::TimeTicks::Now() - start;
+      // Spin with YieldCurrentThread for at most one ms - this ensures maximum
+      // responsiveness. After that spin with Sleep(1ms) so that we don't burn
+      // excessive CPU time - this also avoids infinite loops due to priority
+      // inversions (https://crbug.com/797129).
+      if (elapsed < TimeDelta::FromMilliseconds(1))
+        PlatformThread::YieldCurrentThread();
+      else
+        PlatformThread::Sleep(TimeDelta::FromMilliseconds(1));
+    } while (subtle::Acquire_Load(state) == kLazyInstanceStateCreating);
+  }
+  // Someone else created the instance.
+  return false;
+}
+
+void CompleteLazyInstance(subtle::AtomicWord* state,
+                          subtle::AtomicWord new_instance,
+                          void (*destructor)(void*),
+                          void* destructor_arg) {
+  // Instance is created, go from CREATING to CREATED (or reset it if
+  // |new_instance| is null). Releases visibility over |private_buf_| to
+  // readers. Pairing Acquire_Load is in NeedsLazyInstance().
+  subtle::Release_Store(state, new_instance);
+
+  // Make sure that the lazily instantiated object will get destroyed at exit.
+  if (new_instance && destructor)
+    AtExitManager::RegisterCallback(destructor, destructor_arg);
+}
+
+}  // namespace internal
+}  // namespace base