Renderer reading side of gamepad

content/browser/gamepad/gamepad_provider.cc

 // 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.
 
 #include <cmath>
 #include <set>
 #include <vector>
 
 #include "base/bind.h"
 #include "base/logging.h"
@@ skipping 101 matching lines @@
   // Pass ownership of fetcher to provider_.
   data_fetcher_.swap(provided_fetcher_);
 
   // Start polling.
   ScheduleDoPoll();
 }
 
 void Provider::DoPoll() {
   DCHECK(MessageLoop::current() == polling_thread_->message_loop());
   GamepadHardwareBuffer* hwbuf = SharedMemoryAsHardwareBuffer();
-  base::subtle::Barrier_AtomicIncrement(&hwbuf->start_marker, 1);
+
+  ANNOTATE_BENIGN_RACE_SIZED(
+      &hwbuf->buffer,
+      sizeof(WebKit::WebGamepads),
+      "Racey reads are discarded");
+
+  // Acquire the SeqLock. There is only ever one writer to this data.
+  // See gamepad_hardware_buffer.h.
+  hwbuf->sequence.WriteBegin();
   data_fetcher_->GetGamepadData(&hwbuf->buffer, devices_changed_);
-  base::subtle::Barrier_AtomicIncrement(&hwbuf->end_marker, 1);
+  hwbuf->sequence.WriteEnd();
   devices_changed_ = false;
   // Schedule our next interval of polling.
   ScheduleDoPoll();
 }
 
 void Provider::ScheduleDoPoll() {
   DCHECK(MessageLoop::current() == polling_thread_->message_loop());
 
   MessageLoop::current()->PostDelayedTask(
       FROM_HERE,
       base::Bind(&Provider::DoPoll, weak_factory_.GetWeakPtr()),
       kDesiredSamplingIntervalMs);
 }
 
 GamepadHardwareBuffer* Provider::SharedMemoryAsHardwareBuffer() {
   void* mem = gamepad_shared_memory_.memory();
   DCHECK(mem);
   return static_cast<GamepadHardwareBuffer*>(mem);
 }
 
 }  // namespace gamepad