Revert 143903 - Lock-free GamepadSeqLock

The change
 - provides an improved lock-free SeqLock implementation which eliminates any potential blocking of readers.
 - provides a higher-level and simpler API as was suggested by Darin.
 - ThreadSanitizer report suppressions are replaced with correct synchronization. 
 - eliminates nasty kMaximumContentionCount and associated histogram.

Review URL: https://chromiumcodereview.appspot.com/8772004

TBR=dvyukov@chromium.org
Review URL: https://chromiumcodereview.appspot.com/10656020

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@143914 0039d316-1c4b-4281-b951-d872f2087c98

1 files changed, 25 insertions, 1 deletions

diff --git a/content/renderer/gamepad_shared_memory_reader.cc b/content/renderer/gamepad_shared_memory_reader.cc
index 0d249a7..55c6f62 100644
--- a/content/renderer/gamepad_shared_memory_reader.cc
+++ b/content/renderer/gamepad_shared_memory_reader.cc
@@ -41,7 +41,31 @@ void GamepadSharedMemoryReader::SampleGamepads(WebKit::WebGamepads& gamepads) {
   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.