Clean up condition variable usage in SequencedWorkerPool

base/threading/sequenced_worker_pool.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 "base/threading/sequenced_worker_pool.h"
 
 #include <list>
 #include <map>
 #include <set>
 #include <utility>
@@ skipping 75 matching lines @@
   bool PostTask(const std::string* optional_token_name,
                 SequenceToken sequence_token,
                 WorkerShutdown shutdown_behavior,
                 const tracked_objects::Location& from_here,
                 const Closure& task);
 
   bool RunsTasksOnCurrentThread() const;
 
   void FlushForTesting();
 
+  void TriggerSpuriousWorkSignalForTesting();
+
+  int GetWorkSignalCountForTesting() const;
+
   void Shutdown();
 
   void SetTestingObserver(TestingObserver* observer);
 
   // Runs the worker loop on the background thread.
   void ThreadLoop(Worker* this_worker);
 
  private:
+  // Returns whether there are no more pending tasks and all threads
+  // are idle.  Must be called under lock.
+  bool IsIdle() const;
+
   // Called from within the lock, this converts the given token name into a
   // token ID, creating a new one if necessary.
   int LockedGetNamedTokenID(const std::string& name);
 
   // The calling code should clear the given delete_these_oustide_lock
   // vector the next time the lock is released. See the implementation for
   // a more detailed description.
   bool GetWork(SequencedTask* task,
                std::vector<Closure>* delete_these_outside_lock);
 
@@ skipping 21 matching lines @@
   //
   // See the implementedion for more.
   int PrepareToStartAdditionalThreadIfHelpful();
 
   // The second part of thread creation after
   // PrepareToStartAdditionalThreadIfHelpful with the thread number it
   // generated. This actually creates the thread and should be called outside
   // the lock to avoid blocking important work starting a thread in the lock.
   void FinishStartingAdditionalThread(int thread_number);
 
+  // Signal |has_work_| and increment |has_work_signal_count_|.
+  void SignalHasWork();
+
   // Checks whether there is work left that's blocking shutdown. Must be
   // called inside the lock.
   bool CanShutdown() const;
 
   SequencedWorkerPool* const worker_pool_;
 
   // The last sequence number used. Managed by GetSequenceToken, since this
   // only does threadsafe increment operations, you do not need to hold the
   // lock.
   volatile subtle::Atomic32 last_sequence_number_;
 
   // This lock protects |everything in this class|. Do not read or modify
   // anything without holding this lock. Do not block while holding this
   // lock.
   mutable Lock lock_;
 
-  // Condition variable used to wake up worker threads when a task is runnable.
-  ConditionVariable cond_var_;
+  // Condition variable that is waited on by worker threads until new
+  // tasks are posted or shutdown starts.
+  ConditionVariable has_work_cv_;
+
+  // Number of times |has_work_| has been signalled.  Used for testing.
+  int has_work_signal_count_;
+
+  // Condition variable that is waited on by non-worker threads (in
+  // FlushForTesting()) until IsIdle() goes to true.
+  ConditionVariable is_idle_cv_;
+
+  // Condition variable that is waited on by non-worker threads (in
+  // Shutdown()) until CanShutdown() goes to true.
+  ConditionVariable can_shutdown_cv_;
 
   // The maximum number of worker threads we'll create.
   const size_t max_threads_;
 
   const std::string thread_name_prefix_;
 
   // Associates all known sequence token names with their IDs.
   std::map<std::string, int> named_sequence_tokens_;
 
   // Owning pointers to all threads we've created so far, indexed by
@@ skipping 65 matching lines @@
 
 // Inner definitions ---------------------------------------------------------
 
 SequencedWorkerPool::Inner::Inner(
     SequencedWorkerPool* worker_pool,
     size_t max_threads,
     const std::string& thread_name_prefix)
     : worker_pool_(worker_pool),
       last_sequence_number_(0),
       lock_(),
-      cond_var_(&lock_),
+      has_work_cv_(&lock_),
+      has_work_signal_count_(0),
+      is_idle_cv_(&lock_),
+      can_shutdown_cv_(&lock_),
       max_threads_(max_threads),
       thread_name_prefix_(thread_name_prefix),
       thread_being_created_(false),
       waiting_thread_count_(0),
       blocking_shutdown_thread_count_(0),
       pending_task_count_(0),
       blocking_shutdown_pending_task_count_(0),
       shutdown_called_(false),
       testing_observer_(NULL)  {}
 
@@ skipping 52 matching lines @@
       blocking_shutdown_pending_task_count_++;
 
     create_thread_id = PrepareToStartAdditionalThreadIfHelpful();
   }
 
   // Actually start the additional thread or signal an existing one now that
   // we're outside the lock.
   if (create_thread_id)
     FinishStartingAdditionalThread(create_thread_id);
   else
-    cond_var_.Signal();
+    SignalHasWork();
 
   return true;
 }
 
 bool SequencedWorkerPool::Inner::RunsTasksOnCurrentThread() const {
   AutoLock lock(lock_);
   return ContainsKey(threads_, PlatformThread::CurrentId());
 }
 
 void SequencedWorkerPool::Inner::FlushForTesting() {
-  {
-    AutoLock lock(lock_);
-    while (pending_task_count_ > 0 || waiting_thread_count_ < threads_.size())
-      cond_var_.Wait();
-  }
-  cond_var_.Signal();
+  AutoLock lock(lock_);
+  while (!IsIdle())
+    is_idle_cv_.Wait();
+}
+
+void SequencedWorkerPool::Inner::TriggerSpuriousWorkSignalForTesting() {
+  SignalHasWork();
+}
+
+int SequencedWorkerPool::Inner::GetWorkSignalCountForTesting() const {
+  AutoLock lock(lock_);
+  return has_work_signal_count_;
 }
 
 void SequencedWorkerPool::Inner::Shutdown() {
   // Mark us as terminated and go through and drop all tasks that aren't
   // required to run on shutdown. Since no new tasks will get posted once the
   // terminated flag is set, this ensures that all remaining tasks are required
   // for shutdown whenever the termianted_ flag is set.
   {
     AutoLock lock(lock_);
 
     if (shutdown_called_)
       return;
     shutdown_called_ = true;
 
     // Tickle the threads. This will wake up a waiting one so it will know that
     // it can exit, which in turn will wake up any other waiting ones.
-    cond_var_.Signal();
+    has_work_cv_.Signal();
 
     // There are no pending or running tasks blocking shutdown, we're done.
     if (CanShutdown())
       return;
   }
 
-  // If we get here, we know we're either waiting on a blocking task that's
-  // currently running, waiting on a blocking task that hasn't been scheduled
-  // yet, or both. Block on the "queue empty" event to know when all tasks are
-  // complete. This must be done outside the lock.
+  // If we're here, then something is blocking shutdown.  So wait for
+  // CanShutdown() to go to true.
+
   if (testing_observer_)
     testing_observer_->WillWaitForShutdown();
 
   TimeTicks shutdown_wait_begin = TimeTicks::Now();
 
-  // Wait for no more tasks.
   {
     AutoLock lock(lock_);
     while (!CanShutdown())
-      cond_var_.Wait();
+      can_shutdown_cv_.Wait();
   }
   UMA_HISTOGRAM_TIMES("SequencedWorkerPool.ShutdownDelayTime",
                       TimeTicks::Now() - shutdown_wait_begin);
 }
 
 void SequencedWorkerPool::Inner::SetTestingObserver(
     TestingObserver* observer) {
   AutoLock lock(lock_);
   testing_observer_ = observer;
 }
 
 void SequencedWorkerPool::Inner::ThreadLoop(Worker* this_worker) {
   {
     AutoLock lock(lock_);
     DCHECK(thread_being_created_);
     thread_being_created_ = false;
     std::pair<ThreadMap::iterator, bool> result =
         threads_.insert(
             std::make_pair(this_worker->tid(), make_linked_ptr(this_worker)));
     DCHECK(result.second);
 
     while (true) {
       // See GetWork for what delete_these_outside_lock is doing.
       SequencedTask task;
       std::vector<Closure> delete_these_outside_lock;
       if (GetWork(&task, &delete_these_outside_lock)) {
         int new_thread_id = WillRunWorkerTask(task);
         {
           AutoUnlock unlock(lock_);
-          cond_var_.Signal();
+          // There may be more work available, so wake up another
+          // worker thread. (Technically not required, since we
+          // already get a signal for each new task, but it doesn't
+          // hurt.)
+          has_work_cv_.Signal();
           delete_these_outside_lock.clear();
 
           // Complete thread creation outside the lock if necessary.
           if (new_thread_id)
             FinishStartingAdditionalThread(new_thread_id);
 
           task.task.Run();
 
           // Make sure our task is erased outside the lock for the same reason
           // we do this with delete_these_oustide_lock.
           task.task = Closure();
         }
         DidRunWorkerTask(task);  // Must be done inside the lock.
       } else {
         // When we're terminating and there's no more work, we can
         // shut down.  You can't get more tasks posted once
         // shutdown_called_ is set. There may be some tasks stuck
         // behind running ones with the same sequence token, but
         // additional threads won't help this case.
         if (shutdown_called_)
           break;
         waiting_thread_count_++;
-        cond_var_.Signal();  // For Flush() that may be waiting on the
-                             // waiting thread count to go up.
-        cond_var_.Wait();
+        // This is the only time that IsIdle() can go to true.
+        if (IsIdle())
+          is_idle_cv_.Signal();
+        has_work_cv_.Wait();
         waiting_thread_count_--;
       }
     }
-  }
+  }  // Release lock_.
 
   // We noticed we should exit. Wake up the next worker so it knows it should
   // exit as well (because the Shutdown() code only signals once).
-  cond_var_.Signal();
+  has_work_cv_.Signal();
+
+  // Possibly unblock shutdown.
+  can_shutdown_cv_.Signal();
+}
+
+bool SequencedWorkerPool::Inner::IsIdle() const {
+  lock_.AssertAcquired();
+  return pending_task_count_ == 0 && waiting_thread_count_ == threads_.size();
 }
 
 int SequencedWorkerPool::Inner::LockedGetNamedTokenID(
     const std::string& name) {
   lock_.AssertAcquired();
   DCHECK(!name.empty());
 
   std::map<std::string, int>::const_iterator found =
       named_sequence_tokens_.find(name);
   if (found != named_sequence_tokens_.end())
@@ skipping 63 matching lines @@
       // vector they passed to us once the lock is exited to make this
       // happen.
       delete_these_outside_lock->push_back(i->task);
       i = pending_tasks_.erase(i);
       pending_task_count_--;
     } else {
       // Found a runnable task.
       *task = *i;
       i = pending_tasks_.erase(i);
       pending_task_count_--;
-      if (task->shutdown_behavior == BLOCK_SHUTDOWN)
+      if (task->shutdown_behavior == BLOCK_SHUTDOWN) {
         blocking_shutdown_pending_task_count_--;
+      }
 
       found_task = true;
       break;
     }
   }
 
   // Track the number of tasks we had to skip over to see if we should be
   // making this more efficient. If this number ever becomes large or is
   // frequently "some", we should consider the optimization above.
   UMA_HISTOGRAM_COUNTS_100("SequencedWorkerPool.UnrunnableTaskCount",
@@ skipping 100 matching lines @@
 void SequencedWorkerPool::Inner::FinishStartingAdditionalThread(
     int thread_number) {
   // Called outside of the lock.
   DCHECK(thread_number > 0);
 
   // The worker is assigned to the list when the thread actually starts, which
   // will manage the memory of the pointer.
   new Worker(worker_pool_, thread_number, thread_name_prefix_);
 }
 
+void SequencedWorkerPool::Inner::SignalHasWork() {
+  has_work_cv_.Signal();
+  {
+    AutoLock lock(lock_);
+    ++has_work_signal_count_;
+  }
+}
+
 bool SequencedWorkerPool::Inner::CanShutdown() const {
   lock_.AssertAcquired();
   // See PrepareToStartAdditionalThreadIfHelpful for how thread creation works.
   return !thread_being_created_ &&
          blocking_shutdown_thread_count_ == 0 &&
          blocking_shutdown_pending_task_count_ == 0;
 }
 
 // SequencedWorkerPool --------------------------------------------------------
 
 SequencedWorkerPool::SequencedWorkerPool(
     size_t max_threads,
     const std::string& thread_name_prefix)
     : constructor_message_loop_(MessageLoopProxy::current()),
       inner_(new Inner(ALLOW_THIS_IN_INITIALIZER_LIST(this),
                        max_threads, thread_name_prefix)) {
   DCHECK(constructor_message_loop_.get());
 }
 
 SequencedWorkerPool::~SequencedWorkerPool() {}
 
 void SequencedWorkerPool::OnDestruct() const {
   // TODO(akalin): Once we can easily check if we're on a worker
   // thread or not, use that instead of restricting destruction to
   // only the constructor message loop.
-  if (constructor_message_loop_->BelongsToCurrentThread()) {
+  if (constructor_message_loop_->BelongsToCurrentThread())
     delete this;
-  } else {
+  else
     constructor_message_loop_->DeleteSoon(FROM_HERE, this);
-  }
 }
 
 SequencedWorkerPool::SequenceToken SequencedWorkerPool::GetSequenceToken() {
   return inner_->GetSequenceToken();
 }
 
 SequencedWorkerPool::SequenceToken SequencedWorkerPool::GetNamedSequenceToken(
     const std::string& name) {
   return inner_->GetNamedSequenceToken(name);
 }
@@ skipping 58 matching lines @@
 }
 
 bool SequencedWorkerPool::RunsTasksOnCurrentThread() const {
   return inner_->RunsTasksOnCurrentThread();
 }
 
 void SequencedWorkerPool::FlushForTesting() {
   inner_->FlushForTesting();
 }
 
+void SequencedWorkerPool::TriggerSpuriousWorkSignalForTesting() {
+  inner_->TriggerSpuriousWorkSignalForTesting();
+}
+
+int SequencedWorkerPool::GetWorkSignalCountForTesting() const {
+  return inner_->GetWorkSignalCountForTesting();
+}
+
 void SequencedWorkerPool::Shutdown() {
+  DCHECK(constructor_message_loop_->BelongsToCurrentThread());
   inner_->Shutdown();
 }
 
 void SequencedWorkerPool::SetTestingObserver(TestingObserver* observer) {
   inner_->SetTestingObserver(observer);
 }
 
 }  // namespace base