TaskScheduler: Atomic operations in TaskTracker

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/base_export.h"
 #include "base/callback_forward.h"
 #include "base/macros.h"
 #include "base/metrics/histogram_base.h"
-#include "base/synchronization/condition_variable.h"
+#include "base/synchronization/waitable_event.h"
 #include "base/task_scheduler/scheduler_lock.h"
 #include "base/task_scheduler/task.h"
 #include "base/task_scheduler/task_traits.h"
 
 namespace base {
 namespace internal {
 
 // All tasks go through the scheduler's TaskTracker when they are posted and
 // when they are executed. The TaskTracker enforces shutdown semantics and takes
 // care of tracing and profiling. This class is thread-safe.
@@ skipping 12 matching lines @@
   void Shutdown();
 
   // Informs this TaskTracker that |task| is about to be posted. Returns true if
   // this operation is allowed (|task| should be posted if-and-only-if it is).
   bool WillPostTask(const Task* task);
 
   // Runs |task| unless the current shutdown state prevents that. WillPostTask()
   // must have allowed |task| to be posted.
   void RunTask(const Task* task);
 
+  // Returns true if shutdown has completed.
+  bool ShutdownCompleted() const;
+
   // Returns true while shutdown is in progress (i.e. Shutdown() has been called
   // but hasn't returned).
   bool IsShuttingDownForTesting() const;
 
-  bool shutdown_completed() const {
-    AutoSchedulerLock auto_lock(lock_);
-    return shutdown_completed_;
-  }
+ private:
+  class State;
 
- private:
   // Called before WillPostTask() informs the tracing system that a task has
   // been posted. Updates |num_tasks_blocking_shutdown_| if necessary and
   // returns true if the current shutdown state allows the task to be posted.
   bool BeforePostTask(TaskShutdownBehavior shutdown_behavior);
 
   // Called before a task with |shutdown_behavior| is run by RunTask(). Updates
   // |num_tasks_blocking_shutdown_| if necessary and returns true if the current
   // shutdown state allows the task to be run.
   bool BeforeRunTask(TaskShutdownBehavior shutdown_behavior);
 
   // Called after a task with |shutdown_behavior| has been run by RunTask().
   // Updates |num_tasks_blocking_shutdown_| and signals |shutdown_cv_| if
   // necessary.
   void AfterRunTask(TaskShutdownBehavior shutdown_behavior);
 
-  // Synchronizes access to all members.
-  mutable SchedulerLock lock_;
+  // Called when the number of tasks blocking shutdown becomes zero after
+  // shutdown has started.
+  void OnBlockingShutdownTasksComplete();
 
-  // Condition variable signaled when |num_tasks_blocking_shutdown_| reaches
-  // zero while shutdown is in progress. Null if shutdown isn't in progress.
-  std::unique_ptr<ConditionVariable> shutdown_cv_;
+  // Number of tasks blocking shutdown and boolean indicating whether shutdown
+  // has started.
+  const std::unique_ptr<State> state_;
 
-  // Number of tasks blocking shutdown.
-  size_t num_tasks_blocking_shutdown_ = 0;
+  // Synchronizes access to shutdown related members below.
+  mutable SchedulerLock shutdown_lock_;
+
+  // Event instantiated when shutdown starts and signaled when shutdown
+  // completes.
+  std::unique_ptr<WaitableEvent> shutdown_event_;
 
   // Number of BLOCK_SHUTDOWN tasks posted during shutdown.
   HistogramBase::Sample num_block_shutdown_tasks_posted_during_shutdown_ = 0;
 
-  // True once Shutdown() has returned. No new task can be scheduled after this.
-  bool shutdown_completed_ = false;
-
   DISALLOW_COPY_AND_ASSIGN(TaskTracker);
 };
 
 }  // namespace internal
 }  // namespace base
 
 #endif  // BASE_TASK_SCHEDULER_TASK_TRACKER_H_