Use shared memory to update the renderer's view of the command buffer state.

gpu/command_buffer/common/command_buffer_shared.h

+// 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.
+
+#ifndef GPU_COMMAND_BUFFER_COMMON_COMMAND_BUFFER_SHARED_H_
+#define GPU_COMMAND_BUFFER_COMMON_COMMAND_BUFFER_SHARED_H_
+
+#include "command_buffer.h"
+#include "base/atomicops.h"

[greggman, 2012/02/11 04:17:21]

I don't think you can include base from common  or client as both are used in
untrusted Pepper code I think.  You could if def them out for NaCl if there's a
fallback?

+
+namespace gpu {
+
+// This is a standard 4-slot asynchronous communication mechanism, used to
+// ensure that the reader gets a consistent copy of what the writer wrote.
+template<typename T>
+class SharedState {
+  T states_[2][2];
+  base::subtle::Atomic32 reading_;
+  base::subtle::Atomic32 latest_;
+  base::subtle::Atomic32 slots_[2];
+
+public:
+
+  void Initialize() {
+    for (int i = 0; i < 2; ++i) {
+      for (int j = 0; j < 2; ++j) {
+        states_[i][j] = T();
+      }
+    }
+    base::subtle::NoBarrier_Store(&reading_, 0);
+    base::subtle::NoBarrier_Store(&latest_, 0);
+    base::subtle::NoBarrier_Store(&slots_[0], 0);
+    base::subtle::Acquire_Store(&slots_[1], 0);
+  }
+
+  void Write(const T& state) {
+    int towrite = !base::subtle::Acquire_Load(&reading_);
+    int index = !base::subtle::Acquire_Load(&slots_[towrite]);
+    states_[towrite][index] = state;
+    base::subtle::MemoryBarrier();
+    base::subtle::Acquire_Store(&slots_[towrite], index);
+    base::subtle::Acquire_Store(&latest_, towrite);
+  }
+
+  // Attempt to update the state, updating only if the generation counter is
+  // newer.
+  void Read(T* state) {
+    base::subtle::MemoryBarrier();
+    int toread = !!base::subtle::Acquire_Load(&latest_);
+    base::subtle::Acquire_Store(&reading_, toread);
+    int index = !!base::subtle::Acquire_Load(&slots_[toread]);
+    if (states_[toread][index].generation - state->generation < 0x80000000U)
+      *state = states_[toread][index];
+  }
+};
+
+typedef SharedState<CommandBuffer::State> CommandBufferSharedState;
+
+}  // namespace gpu
+
+#endif  // GPU_COMMAND_BUFFER_COMMON_COMMAND_BUFFER_SHARED_H_