gpu: Thread-safe command buffer state lookup.

gpu/command_buffer/client/ring_buffer.cc

Index: gpu/command_buffer/client/ring_buffer.cc
diff --git a/gpu/command_buffer/client/ring_buffer.cc b/gpu/command_buffer/client/ring_buffer.cc
index ebbb1182aed5f175f8391de22e79118cdf6f4b71..118dc57276e5d79d4810c8bcdb34561da70bb2fd 100644
--- a/gpu/command_buffer/client/ring_buffer.cc
+++ b/gpu/command_buffer/client/ring_buffer.cc
@@ -146,10 +146,9 @@ void RingBuffer::DiscardBlock(void* pointer) {
 }
 
 unsigned int RingBuffer::GetLargestFreeSizeNoWaiting() {
-  unsigned int last_token_read = helper_->last_token_read();
   while (!blocks_.empty()) {
     Block& block = blocks_.front();
-    if (block.token > last_token_read || block.state == IN_USE) break;
+    if (!helper_->HasTokenPassed(block.token) || block.state == IN_USE) break;

[jbauman, 2016/12/09 03:16:03]

This will most likely hit the lock at least once per call to RingBuffer::Alloc,
as if any ringbuffer entry is being used on the service side it'll free blocks
until it hits that and has to check its token. That would probably be
acceptable, and I suspect the perf bots will let us know if not.

[sunnyps, 2016/12/09 04:09:37]

I didn't expose token from the helper directly because this wasn't handling
token wrap around and I wanted to avoid a similar mistake in the future.
HasTokenPassed has an early out if the cached token >= given token.

     FreeOldestBlock();
   }
   if (free_offset_ == in_use_offset_) {