Synchronize memory in TaskTracker::Flush().

base/task_scheduler/task_tracker.h

 // Copyright 2016 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.
 
 #ifndef BASE_TASK_SCHEDULER_TASK_TRACKER_H_
 #define BASE_TASK_SCHEDULER_TASK_TRACKER_H_
 
 #include <memory>
 
 #include "base/atomicops.h"
@@ skipping 99 matching lines @@
 
   // Records the TaskScheduler.TaskLatency.[task priority].[may block] histogram
   // for |task|.
   void RecordTaskLatencyHistogram(Task* task);
 
   // Number of tasks blocking shutdown and boolean indicating whether shutdown
   // has started.
   const std::unique_ptr<State> state_;
 
   // Number of undelayed tasks that haven't completed their execution. Is
-  // incremented and decremented without a barrier. When it reaches zero,
-  // |flush_lock_| is acquired (forcing memory synchronization) and |flush_cv_|
-  // is signaled.
+  // decremented with a memory barrier after a task runs. Is accessed with an
+  // acquire memory barrier in Flush(). The memory barriers ensure that the
+  // memory written by flushed tasks is visible when Flush() returns.
   subtle::Atomic32 num_pending_undelayed_tasks_ = 0;
 
   // Lock associated with |flush_cv_|. Partially synchronizes access to
   // |num_pending_undelayed_tasks_|. Full synchronization isn't needed because
   // it's atomic, but synchronization is needed to coordinate waking and
   // sleeping at the right time.
   mutable SchedulerLock flush_lock_;
 
   // Signaled when |num_pending_undelayed_tasks_| is zero or when shutdown
   // completes.
@@ skipping 15 matching lines @@
   // Number of BLOCK_SHUTDOWN tasks posted during shutdown.
   HistogramBase::Sample num_block_shutdown_tasks_posted_during_shutdown_ = 0;
 
   DISALLOW_COPY_AND_ASSIGN(TaskTracker);
 };
 
 }  // namespace internal
 }  // namespace base
 
 #endif  // BASE_TASK_SCHEDULER_TASK_TRACKER_H_