[Reland 1] Pepper takes ownership of a mailbox before passing it to the texture layer.

gpu/ipc/service/gpu_channel.cc

 // Copyright (c) 2012 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 "gpu/ipc/service/gpu_channel.h"
 
 #include <utility>
 
 #if defined(OS_WIN)
 #include <windows.h>
@@ skipping 222 matching lines @@
   if (scheduled_)
     channel_->PostHandleMessage(this);
 
   sync_point_order_data_->PauseProcessingOrderNumber(
       channel_messages_.front()->order_number);
 }
 
 void GpuChannelMessageQueue::FinishMessageProcessing() {
   base::AutoLock auto_lock(channel_lock_);
   DCHECK(!channel_messages_.empty());
-  DCHECK(scheduled_);

[piman, 2016/05/03 20:57:52]

Oh, this can't go away. Actually we need to not get here. 
The logic in GpuChannel::HandleMessage needs to go to the PauseMessageProcessing
path. Let me comment there.

[sunnyps, 2016/05/03 21:17:10]

Yes, this DCHECK is needed. We shouldn't finish a message if we get
descheduled(fence sync)/preempted.

 
   sync_point_order_data_->FinishProcessingOrderNumber(
       channel_messages_.front()->order_number);
   channel_messages_.pop_front();
 
   if (!channel_messages_.empty())
     channel_->PostHandleMessage(this);
 
   if (preempting_flag_) {
     io_task_runner_->PostTask(
@@ skipping 531 matching lines @@
   GpuCommandBufferStub* stub = stubs_.get(routing_id);
 
   DCHECK(!stub || stub->IsScheduled());
 
   DVLOG(1) << "received message @" << &msg << " on channel @" << this
            << " with type " << msg.type();
 
   HandleMessageHelper(msg);
 
   // If we get descheduled or yield while processing a message.
   if (stub && stub->HasUnprocessedCommands()) {

[piman, 2016/05/03 20:57:52]

I think the stub->HasUnprocessedCommands() check should be replaced by
!stub->IsScheduled(), and the DCHECK below account for both AsyncFlush as well
as WaitSyncToken.

[sunnyps, 2016/05/03 21:17:10]

Yeah, the previous assumption was that the stub only gets descheduled while
running commands (AsyncFlush) but with the WaitFenceSync IPC this is no longer
true. This should be changed to message_queue->scheduled() - a good followup
would be to merge the PauseMessageProcessing/FinishMessageProcessing methods.
Also we do want to keep the DCHECK for AsyncFlush if
stub->HasUnprocessedCommands.

[erikchen, 2016/05/04 16:20:35]

I replaced the conditional with if (!message_queue->IsScheduled()) { and updated
the DCHECK.

     DCHECK_EQ((uint32_t)GpuCommandBufferMsg_AsyncFlush::ID, msg.type());
     message_queue->PauseMessageProcessing();
   } else {
     message_queue->FinishMessageProcessing();
   }
 }
 
 void GpuChannel::HandleMessageHelper(const IPC::Message& msg) {
   int32_t routing_id = msg.routing_id();
 
@@ skipping 243 matching lines @@
           ->CreateImageForGpuMemoryBuffer(handle,
                                           size,
                                           format,
                                           internalformat,
                                           client_id_);
     }
   }
 }
 
 }  // namespace gpu