gpu: Avoid holding onto lock while destroy Sequence

There are reports that destroying Sequence can be expensive, and lock up
other threads. So destroy it outside of the gpu::Scheduler lock.

Bug: 916977
Change-Id: I774cf8188e81f5ebed5fd37edd45278b415df7cb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2277110
Commit-Queue: Bo <boliu@chromium.org>
Reviewed-by: Sunny Sachanandani <sunnyps@chromium.org>
Cr-Commit-Position: refs/heads/master@{#785764}

gpu/command_buffer/service/scheduler.cc

@@ -327,14 +327,18 @@ SequenceId Scheduler::CreateSequence(SchedulingPriority priority) {
 }
 
 void Scheduler::DestroySequence(SequenceId sequence_id) {
-  base::AutoLock auto_lock(lock_);
+  base::circular_deque<Sequence::Task> tasks_to_be_destroyed;
+  {
+    base::AutoLock auto_lock(lock_);
 
-  Sequence* sequence = GetSequence(sequence_id);
-  DCHECK(sequence);
-  if (sequence->scheduled())
-    rebuild_scheduling_queue_ = true;
+    Sequence* sequence = GetSequence(sequence_id);
+    DCHECK(sequence);
+    if (sequence->scheduled())
+      rebuild_scheduling_queue_ = true;
 
-  sequences_.erase(sequence_id);
+    tasks_to_be_destroyed = std::move(sequence->tasks_);
+    sequences_.erase(sequence_id);
+  }
 }
 
 Scheduler::Sequence* Scheduler::GetSequence(SequenceId sequence_id) {

gpu/command_buffer/service/scheduler.h

@@ -200,6 +200,8 @@ class GPU_EXPORT Scheduler {
     SchedulingPriority current_priority() const { return current_priority_; }
 
    private:
+    friend class Scheduler;
+
     enum RunningState { IDLE, SCHEDULED, RUNNING };
 
     struct WaitFence {