TaskScheduler: Atomic operations in TaskTracker

base/task_scheduler/task_tracker.h

Index: base/task_scheduler/task_tracker.h
diff --git a/base/task_scheduler/task_tracker.h b/base/task_scheduler/task_tracker.h
index 795ae4937058d0a8f24719b9ae0edfa3641e0b5f..fc0901f7c139ade379e6e334a3aa7603efec7b49 100644
--- a/base/task_scheduler/task_tracker.h
+++ b/base/task_scheduler/task_tracker.h
@@ -11,7 +11,7 @@
 #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"
@@ -44,16 +44,16 @@ class BASE_EXPORT TaskTracker {
   // must have allowed |task| to be posted.
   void RunTask(const Task* task);
 
+  // Returns true if shutdown has completed.
+  bool IsShutdownComplete() 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;
+
   // 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.
@@ -69,22 +69,24 @@ class BASE_EXPORT TaskTracker {
   // 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);
 };