Revert 146792 - Lock-free GamepadSeqLock (try 2)

content/renderer/gamepad_shared_memory_reader.cc

 // Copyright (c) 2012 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 "content/renderer/gamepad_shared_memory_reader.h"
 
 #include "base/debug/trace_event.h"
 #include "base/metrics/histogram.h"
 #include "content/common/gamepad_messages.h"
 #include "content/public/renderer/render_thread.h"
@@ skipping 23 matching lines @@
       static_cast<GamepadHardwareBuffer*>(memory);
 }
 
 void GamepadSharedMemoryReader::SampleGamepads(WebKit::WebGamepads& gamepads) {
   WebKit::WebGamepads read_into;
   TRACE_EVENT0("GAMEPAD", "SampleGamepads");
 
   if (!base::SharedMemory::IsHandleValid(renderer_shared_memory_handle_))
     return;
 
-  gamepad_hardware_buffer_->gamepads.ReadTo(&gamepads);
+  // Only try to read this many times before failing to avoid waiting here
+  // very long in case of contention with the writer. TODO(scottmg) Tune this
+  // number (as low as 1?) if histogram shows distribution as mostly
+  // 0-and-maximum.
+  const int kMaximumContentionCount = 10;
+  int contention_count = -1;
+  base::subtle::Atomic32 version;
+  do {
+    version = gamepad_hardware_buffer_->sequence.ReadBegin();
+    memcpy(&read_into, &gamepad_hardware_buffer_->buffer, sizeof(read_into));
+    ++contention_count;
+    if (contention_count == kMaximumContentionCount)
+      break;
+  } while (gamepad_hardware_buffer_->sequence.ReadRetry(version));
+  UMA_HISTOGRAM_COUNTS("Gamepad.ReadContentionCount", contention_count);
+
+  if (contention_count >= kMaximumContentionCount) {
+      // We failed to successfully read, presumably because the hardware
+      // thread was taking unusually long. Don't copy the data to the output
+      // buffer, and simply leave what was there before.
+      return;
+  }
+
+  // New data was read successfully, copy it into the output buffer.
+  memcpy(&gamepads, &read_into, sizeof(gamepads));
 
   // Override the "connected" with false until the user has interacted
   // with the gamepad. This is to prevent fingerprinting on drive-by pages.
   for (unsigned i = 0; i < WebKit::WebGamepads::itemsLengthCap; ++i) {
     WebKit::WebGamepad& pad = gamepads.items[i];
     // If the device is physically connected, then check if we should
     // keep it disabled. We track if any of the primary 4 buttons have been
     // pressed to determine a reasonable intentional interaction from the user.
     if (pad.connected) {
       if (ever_interacted_with_[i])
         continue;
       const unsigned kPrimaryInteractionButtons = 4;
       for (unsigned j = 0; j < kPrimaryInteractionButtons; ++j)
         ever_interacted_with_[i] |= pad.buttons[j] > 0.5f;
       // If we've not previously set, and the user still hasn't touched
       // these buttons, then don't pass the data on to the Chromium port.
       if (!ever_interacted_with_[i])
         pad.connected = false;
     }
   }
 }
 
 GamepadSharedMemoryReader::~GamepadSharedMemoryReader() {
   RenderThread::Get()->Send(new GamepadHostMsg_StopPolling());
 }
 
 } // namespace content