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;

[piman, 2016/12/07 20:30:12]

So, this would call GetLastState(), take the lock and do the TryUpdateState for
every single block, which seems very heavyweight for a call that is meant to be
reasonably fast.
Could we grab the latest token only once and then compare against it in the
loop?

[sunnyps, 2016/12/08 01:14:16]

I made HasTokenPassed faster by using the cached token first and then updating
the state if necessary. I don't like exposing the token directly because clients
might not perform the wrap-around check that HasTokenPassed does. Also, clients
have no way of doing the cached token early out.

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