gpu: Thread-safe command buffer state lookup.

gpu/command_buffer/client/gpu_control.h

Index: gpu/command_buffer/client/gpu_control.h
diff --git a/gpu/command_buffer/client/gpu_control.h b/gpu/command_buffer/client/gpu_control.h
index afdb93adee484d5eeb86e6b06751cdddd0e2c17a..62429898c882dcff05f3556d759f9a19dc72f888 100644
--- a/gpu/command_buffer/client/gpu_control.h
+++ b/gpu/command_buffer/client/gpu_control.h
@@ -82,18 +82,29 @@ class GPU_EXPORT GpuControl {
   virtual CommandBufferId GetCommandBufferID() const = 0;
   virtual int32_t GetExtraCommandBufferData() const = 0;
 
-  // Fence Syncs use release counters at a context level, these fence syncs
-  // need to be flushed before they can be shared with other contexts across
-  // channels. Subclasses should implement these functions and take care of
-  // figuring out when a fence sync has been flushed. The difference between
-  // IsFenceSyncFlushed and IsFenceSyncFlushReceived, one is testing is the
-  // client has issued the flush, and the other is testing if the service
-  // has received the flush.
+  // Generates a fence sync which should be inserted into the GL command stream.
+  // When the service executes the fence sync it is released. Fence syncs are
+  // shared with other contexts as sync tokens which encapsulate the fence sync
+  // and the command buffer on which it was generated. Fence syncs need to be
+  // flushed before they can be used by other contexts. Furthermore, the flush
+  // must be verified before sending a sync token across channel boundaries.
   virtual uint64_t GenerateFenceSyncRelease() = 0;
+
+  // Returns true if the fence sync is valid.
   virtual bool IsFenceSyncRelease(uint64_t release) = 0;
+
+  // Returns true if the client has flushed the fence sync.
   virtual bool IsFenceSyncFlushed(uint64_t release) = 0;
+
+  // Returns true if the service has received the fence sync. Used for verifying
+  // sync tokens.
   virtual bool IsFenceSyncFlushReceived(uint64_t release) = 0;
 
+  // Returns true if the service has released (executed) the fence sync. Some
+  // implementations may support calling this from any thread without holding
+  // the lock provided by the client.

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

Are there implementations that are not thread safe?
I think that would be problematic, because GLES2Implementation assumes it is -
at least if we want to claim that ContextSupport::IsFenceSyncReleased is
thread-safe. Do we need to add some introspection
(GpuControl::HasThreadSafeSupport()) so that we can at least DCHECK?

[sunnyps, 2016/12/08 01:14:15]

I made the InProcessCommandBuffer implementation thread-safe. Ppapi and test
only implementations, GLManager and MockClientGpuControl don't implement this
but they don't implement SetLock and a bunch of other things.

+  virtual bool IsFenceSyncReleased(uint64_t release) = 0;
+
   // Runs |callback| when sync token is signalled.
   virtual void SignalSyncToken(const SyncToken& sync_token,
                                const base::Closure& callback) = 0;