zero-copy: Clarify to allocate/destroy GpuMemoryBuffer on any thread and use it on the main thread o

content/common/gpu/gpu_memory_buffer_factory_io_surface.cc

 // Copyright 2014 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/common/gpu/gpu_memory_buffer_factory_io_surface.h"
 
 #include <CoreFoundation/CoreFoundation.h>
 
 #include "base/logging.h"
 #include "ui/gl/gl_image_io_surface.h"
@@ skipping 141 matching lines @@
 }
 
 scoped_refptr<gfx::GLImage>
 GpuMemoryBufferFactoryIOSurface::CreateImageForGpuMemoryBuffer(
     const gfx::GpuMemoryBufferHandle& handle,
     const gfx::Size& size,
     gfx::GpuMemoryBuffer::Format format,
     unsigned internalformat,
     int client_id) {
   base::AutoLock lock(io_surfaces_lock_);
-
+  DCHECK(thread_checker_.CalledOnValidThread());

[reveman, 2015/04/07 13:07:05]

I don't think we want a thread checker here as that prevents this function from
being called on any thread.

[dshwang, 2015/04/07 14:04:26]

We allow CreateGpuMemoryBuffer() to be called on any thread, but we allow
CreateImageForGpuMemoryBuffer() to be called on only main thread because only
client API can create GLImage as following call stack

#2 0x7fe562138d61
content::GpuMemoryBufferFactoryIOSurface::CreateImageForGpuMemoryBuffer()
#3 0x7fe562bd608f content::GpuChannel::CreateImageForGpuMemoryBuffer()
#4 0x7fe562be06f2 content::GpuCommandBufferStub::OnCreateImage()
#5 0x7fe562be5ad5 content::GpuCommandBufferStub::OnMessageReceived()

[reveman, 2015/04/07 14:22:00]

Sorry I misread this change.

Yes, image creation is always happening on the same thread. However, there's
nothing about this code that restricts that. Wouldn't it make more sense to have
this thread checker in the GLImage implementations?  The GLImage implementations
with this restriction.

   DCHECK_EQ(handle.type, gfx::IO_SURFACE_BUFFER);
   IOSurfaceMapKey key(handle.id, client_id);
   IOSurfaceMap::iterator it = io_surfaces_.find(key);
   if (it == io_surfaces_.end())
     return scoped_refptr<gfx::GLImage>();
 
   scoped_refptr<gfx::GLImageIOSurface> image(new gfx::GLImageIOSurface(size));
   if (!image->Initialize(it->second.get()))
     return scoped_refptr<gfx::GLImage>();
 
   return image;
 }
 
 }  // namespace content