Re-land: cc: Cancel and re-prioritize worker pool tasks.

cc/base/worker_pool.cc

Index: cc/base/worker_pool.cc
diff --git a/cc/base/worker_pool.cc b/cc/base/worker_pool.cc
index 59dc828980e9b211b415ae95c1bd55d3a65458a3..bbf51c227b89b6b90263678f4c970e49f0b6914a 100644
--- a/cc/base/worker_pool.cc
+++ b/cc/base/worker_pool.cc
@@ -4,46 +4,93 @@
 
 #include "cc/base/worker_pool.h"
 
-#include <algorithm>
+#if defined(OS_ANDROID)
+// TODO(epenner): Move thread priorities to base. (crbug.com/170549)
+#include <sys/resource.h>
+#endif
+
+#include <map>
 
 #include "base/bind.h"
 #include "base/debug/trace_event.h"
+#include "base/hash_tables.h"
 #include "base/stringprintf.h"
-#include "base/synchronization/condition_variable.h"
 #include "base/threading/simple_thread.h"
 #include "base/threading/thread_restrictions.h"
+#include "cc/base/scoped_ptr_deque.h"
+#include "cc/base/scoped_ptr_hash_map.h"
+
+#if defined(COMPILER_GCC)
+namespace BASE_HASH_NAMESPACE {
+template <> struct hash<cc::internal::WorkerPoolTask*> {
+  size_t operator()(cc::internal::WorkerPoolTask* ptr) const {
+    return hash<size_t>()(reinterpret_cast<size_t>(ptr));
+  }
+};
+}  // namespace BASE_HASH_NAMESPACE
+#endif  // COMPILER
 
 namespace cc {
 
-namespace {
-
-class WorkerPoolTaskImpl : public internal::WorkerPoolTask {
- public:
-  WorkerPoolTaskImpl(const WorkerPool::Callback& task,
-                     const base::Closure& reply)
-      : internal::WorkerPoolTask(reply),
-        task_(task) {}
+namespace internal {
 
-  virtual void RunOnThread(unsigned thread_index) OVERRIDE {
-    task_.Run();
-  }
+WorkerPoolTask::WorkerPoolTask()
+    : did_schedule_(false),
+      did_run_(false),
+      did_complete_(false) {
+}
 
- private:
-  WorkerPool::Callback task_;
-};
+WorkerPoolTask::WorkerPoolTask(TaskVector* dependencies)
+    : did_schedule_(false),
+      did_run_(false),
+      did_complete_(false) {
+  dependencies_.swap(*dependencies);
+}
 
-}  // namespace
+WorkerPoolTask::~WorkerPoolTask() {
+  DCHECK_EQ(did_schedule_, did_complete_);
+  DCHECK(!did_run_ || did_schedule_);
+  DCHECK(!did_run_ || did_complete_);
+}
 
-namespace internal {
+void WorkerPoolTask::DidSchedule() {
+  DCHECK(!did_complete_);
+  did_schedule_ = true;
+}
 
-WorkerPoolTask::WorkerPoolTask(const base::Closure& reply) : reply_(reply) {
+void WorkerPoolTask::WillRun() {
+  DCHECK(did_schedule_);
+  DCHECK(!did_complete_);
+  DCHECK(!did_run_);
 }
 
-WorkerPoolTask::~WorkerPoolTask() {
+void WorkerPoolTask::DidRun() {
+  did_run_ = true;
 }
 
 void WorkerPoolTask::DidComplete() {
-  reply_.Run();
+  DCHECK(did_schedule_);
+  DCHECK(!did_complete_);
+  did_complete_ = true;
+}
+
+bool WorkerPoolTask::IsReadyToRun() const {
+  // TODO(reveman): Use counter to improve performance.
+  for (TaskVector::const_reverse_iterator it = dependencies_.rbegin();
+       it != dependencies_.rend(); ++it) {
+    WorkerPoolTask* dependency = *it;
+    if (!dependency->HasFinishedRunning())
+      return false;
+  }
+  return true;
+}
+
+bool WorkerPoolTask::HasFinishedRunning() const {
+  return did_run_;
+}
+
+bool WorkerPoolTask::HasCompleted() const {
+  return did_complete_;
 }
 
 }  // namespace internal
@@ -60,17 +107,51 @@ class WorkerPool::Inner : public base::DelegateSimpleThread::Delegate {
 
   void Shutdown();
 
-  void PostTask(scoped_ptr<internal::WorkerPoolTask> task);
+  // Schedule running of |root| task and all its dependencies. Tasks
+  // previously scheduled but no longer needed to run |root| will be
+  // canceled unless already running. Canceled tasks are moved to
+  // |completed_tasks_| without being run. The result is that once
+  // scheduled, a task is guaranteed to end up in the |completed_tasks_|
+  // queue even if they later get canceled by another call to
+  // ScheduleTasks().
+  void ScheduleTasks(internal::WorkerPoolTask* root);
 
-  // Appends all completed tasks to worker pool's completed tasks queue
-  // and returns true if idle.
-  bool CollectCompletedTasks();
+  // Collect all completed tasks in |completed_tasks|. Returns true if idle.
+  bool CollectCompletedTasks(TaskDeque* completed_tasks);
 
  private:
-  // Appends all completed tasks to |completed_tasks|. Lock must
-  // already be acquired before calling this function.
-  bool AppendCompletedTasksWithLockAcquired(
-      ScopedPtrDeque<internal::WorkerPoolTask>* completed_tasks);
+  class ScheduledTask {
+   public:
+    ScheduledTask(internal::WorkerPoolTask* dependent, unsigned priority)
+        : priority_(priority) {
+      if (dependent)
+        dependents_.push_back(dependent);
+    }
+
+    internal::WorkerPoolTask::TaskVector& dependents() { return dependents_; }
+    unsigned priority() const { return priority_; }
+
+   private:
+    internal::WorkerPoolTask::TaskVector dependents_;
+    unsigned priority_;
+  };
+  typedef internal::WorkerPoolTask* ScheduledTaskMapKey;
+  typedef ScopedPtrHashMap<ScheduledTaskMapKey, ScheduledTask>
+      ScheduledTaskMap;
+
+  // This builds a ScheduledTaskMap from a root task.
+  static unsigned BuildScheduledTaskMapRecursive(
+      internal::WorkerPoolTask* task,
+      internal::WorkerPoolTask* dependent,
+      unsigned priority,
+      ScheduledTaskMap* scheduled_tasks);
+  static void BuildScheduledTaskMap(
+      internal::WorkerPoolTask* root, ScheduledTaskMap* scheduled_tasks);
+
+  // Collect all completed tasks by swapping the contents of
+  // |completed_tasks| and |completed_tasks_|. Lock must be acquired
+  // before calling this function. Returns true if idle.
+  bool CollectCompletedTasksWithLockAcquired(TaskDeque* completed_tasks);
 
   // Schedule an OnIdleOnOriginThread callback if not already pending.
   // Lock must already be acquired before calling this function.
@@ -90,8 +171,8 @@ class WorkerPool::Inner : public base::DelegateSimpleThread::Delegate {
   mutable base::Lock lock_;
 
   // Condition variable that is waited on by worker threads until new
-  // tasks are posted or shutdown starts.
-  base::ConditionVariable has_pending_tasks_cv_;
+  // tasks are ready to run or shutdown starts.
+  base::ConditionVariable has_ready_to_run_tasks_cv_;
 
   // Target message loop used for posting callbacks.
   scoped_refptr<base::MessageLoopProxy> origin_loop_;
@@ -106,15 +187,25 @@ class WorkerPool::Inner : public base::DelegateSimpleThread::Delegate {
   // loop index is 0.
   unsigned next_thread_index_;
 
-  // Number of tasks currently running.
-  unsigned running_task_count_;
-
   // Set during shutdown. Tells workers to exit when no more tasks
   // are pending.
   bool shutdown_;
 
-  typedef ScopedPtrDeque<internal::WorkerPoolTask> TaskDeque;
-  TaskDeque pending_tasks_;
+  // The root task that is a dependent of all other tasks.
+  scoped_refptr<internal::WorkerPoolTask> root_;
+
+  // This set contains all pending tasks.
+  ScheduledTaskMap pending_tasks_;
+
+  // Ordered set of tasks that are ready to run.
+  // TODO(reveman): priority_queue might be more efficient.
+  typedef std::map<unsigned, internal::WorkerPoolTask*> TaskMap;
+  TaskMap ready_to_run_tasks_;
+
+  // This set contains all currently running tasks.
+  ScheduledTaskMap running_tasks_;
+
+  // Completed tasks not yet collected by origin thread.
   TaskDeque completed_tasks_;
 
   ScopedPtrDeque<base::DelegateSimpleThread> workers_;
@@ -127,25 +218,24 @@ WorkerPool::Inner::Inner(WorkerPool* worker_pool,
                          const std::string& thread_name_prefix)
     : worker_pool_on_origin_thread_(worker_pool),
       lock_(),
-      has_pending_tasks_cv_(&lock_),
+      has_ready_to_run_tasks_cv_(&lock_),
       origin_loop_(base::MessageLoopProxy::current()),
       weak_ptr_factory_(this),
       on_idle_callback_(base::Bind(&WorkerPool::Inner::OnIdleOnOriginThread,
                                    weak_ptr_factory_.GetWeakPtr())),
       on_idle_pending_(false),
       next_thread_index_(0),
-      running_task_count_(0),
       shutdown_(false) {
   base::AutoLock lock(lock_);
 
   while (workers_.size() < num_threads) {
     scoped_ptr<base::DelegateSimpleThread> worker = make_scoped_ptr(
         new base::DelegateSimpleThread(
-          this,
-          thread_name_prefix +
-          base::StringPrintf(
-              "Worker%u",
-              static_cast<unsigned>(workers_.size() + 1)).c_str()));
+            this,
+            thread_name_prefix +
+            base::StringPrintf(
+                "Worker%u",
+                static_cast<unsigned>(workers_.size() + 1)).c_str()));
     worker->Start();
     workers_.push_back(worker.Pass());
   }
@@ -156,12 +246,10 @@ WorkerPool::Inner::~Inner() {
 
   DCHECK(shutdown_);
 
-  // Cancel all pending callbacks.
-  weak_ptr_factory_.InvalidateWeakPtrs();
-
   DCHECK_EQ(0u, pending_tasks_.size());
+  DCHECK_EQ(0u, ready_to_run_tasks_.size());
+  DCHECK_EQ(0u, running_tasks_.size());
   DCHECK_EQ(0u, completed_tasks_.size());
-  DCHECK_EQ(0u, running_task_count_);
 }
 
 void WorkerPool::Inner::Shutdown() {
@@ -173,7 +261,7 @@ void WorkerPool::Inner::Shutdown() {
 
     // Wake up a worker so it knows it should exit. This will cause all workers
     // to exit as each will wake up another worker before exiting.
-    has_pending_tasks_cv_.Signal();
+    has_ready_to_run_tasks_cv_.Signal();
   }
 
   while (workers_.size()) {
@@ -183,32 +271,100 @@ void WorkerPool::Inner::Shutdown() {
     base::ThreadRestrictions::ScopedAllowIO allow_io;
     worker->Join();
   }
+
+  // Cancel any pending OnIdle callback.
+  weak_ptr_factory_.InvalidateWeakPtrs();
 }
 
-void WorkerPool::Inner::PostTask(scoped_ptr<internal::WorkerPoolTask> task) {
-  base::AutoLock lock(lock_);
+void WorkerPool::Inner::ScheduleTasks(internal::WorkerPoolTask* root) {
+  // It is OK to call ScheduleTasks() after shutdown if |root| is NULL.
+  DCHECK(!root || !shutdown_);
+
+  scoped_refptr<internal::WorkerPoolTask> new_root(root);
+
+  ScheduledTaskMap new_pending_tasks;
+  ScheduledTaskMap new_running_tasks;
+  TaskMap new_ready_to_run_tasks;
+
+  // Build scheduled task map before acquiring |lock_|.
+  if (root)
+    BuildScheduledTaskMap(root, &new_pending_tasks);
+
+  {
+    base::AutoLock lock(lock_);
+
+    // First remove all completed tasks from |new_pending_tasks|.
+    for (TaskDeque::iterator it = completed_tasks_.begin();
+         it != completed_tasks_.end(); ++it) {
+      internal::WorkerPoolTask* task = *it;
+      new_pending_tasks.take_and_erase(task);
+    }
 
-  pending_tasks_.push_back(task.Pass());
+    // Move tasks not present in |new_pending_tasks| to |completed_tasks_|.
+    for (ScheduledTaskMap::iterator it = pending_tasks_.begin();
+         it != pending_tasks_.end(); ++it) {
+      internal::WorkerPoolTask* task = it->first;
 
-  // There is more work available, so wake up worker thread.
-  has_pending_tasks_cv_.Signal();
+      // Task has completed if not present in |new_pending_tasks|.
+      if (!new_pending_tasks.contains(task))
+        completed_tasks_.push_back(task);
+    }
+
+    // Build new running task set.
+    for (ScheduledTaskMap::iterator it = running_tasks_.begin();
+         it != running_tasks_.end(); ++it) {
+      internal::WorkerPoolTask* task = it->first;
+      // Transfer scheduled task value from |new_pending_tasks| to
+      // |new_running_tasks| if currently running. Value must be set to
+      // NULL if |new_pending_tasks| doesn't contain task. This does
+      // the right in both cases.
+      new_running_tasks.set(task, new_pending_tasks.take_and_erase(task));
+    }
+
+    // Build new "ready to run" tasks queue.
+    for (ScheduledTaskMap::iterator it = new_pending_tasks.begin();
+         it != new_pending_tasks.end(); ++it) {
+      internal::WorkerPoolTask* task = it->first;
+
+      // Completed tasks should not exist in |new_pending_tasks|.
+      DCHECK(!task->HasFinishedRunning());
+
+      // Call DidSchedule() to indicate that this task has been scheduled.
+      // Note: This is only for debugging purposes.
+      task->DidSchedule();
+
+      DCHECK_EQ(0u, new_ready_to_run_tasks.count(it->second->priority()));
+      if (task->IsReadyToRun())
+        new_ready_to_run_tasks[it->second->priority()] = task;
+    }
+
+    // Swap root taskand task sets.
+    // Note: old tasks are intentionally destroyed after releasing |lock_|.
+    root_.swap(new_root);
+    pending_tasks_.swap(new_pending_tasks);
+    running_tasks_.swap(new_running_tasks);
+    ready_to_run_tasks_.swap(new_ready_to_run_tasks);
+
+    // If there is more work available, wake up worker thread.
+    if (!ready_to_run_tasks_.empty())
+      has_ready_to_run_tasks_cv_.Signal();
+  }
 }
 
-bool WorkerPool::Inner::CollectCompletedTasks() {
+bool WorkerPool::Inner::CollectCompletedTasks(TaskDeque* completed_tasks) {
   base::AutoLock lock(lock_);
 
-  return AppendCompletedTasksWithLockAcquired(
-      &worker_pool_on_origin_thread_->completed_tasks_);
+  return CollectCompletedTasksWithLockAcquired(completed_tasks);
 }
 
-bool WorkerPool::Inner::AppendCompletedTasksWithLockAcquired(
-    ScopedPtrDeque<internal::WorkerPoolTask>* completed_tasks) {
+bool WorkerPool::Inner::CollectCompletedTasksWithLockAcquired(
+    TaskDeque* completed_tasks) {
   lock_.AssertAcquired();
 
-  while (completed_tasks_.size())
-    completed_tasks->push_back(completed_tasks_.take_front().Pass());
+  DCHECK_EQ(0u, completed_tasks->size());
+  completed_tasks->swap(completed_tasks_);
 
-  return !running_task_count_ && pending_tasks_.empty();
+  return running_tasks_.empty() && pending_tasks_.empty();
 }
 
 void WorkerPool::Inner::ScheduleOnIdleWithLockAcquired() {
@@ -221,6 +377,8 @@ void WorkerPool::Inner::ScheduleOnIdleWithLockAcquired() {
 }
 
 void WorkerPool::Inner::OnIdleOnOriginThread() {
+  TaskDeque completed_tasks;
+
   {
     base::AutoLock lock(lock_);
 
@@ -228,14 +386,13 @@ void WorkerPool::Inner::OnIdleOnOriginThread() {
     on_idle_pending_ = false;
 
     // Early out if no longer idle.
-    if (running_task_count_ || !pending_tasks_.empty())
+    if (!running_tasks_.empty() || !pending_tasks_.empty())
       return;
 
-    AppendCompletedTasksWithLockAcquired(
-        &worker_pool_on_origin_thread_->completed_tasks_);
+    CollectCompletedTasksWithLockAcquired(&completed_tasks);
   }
 
-  worker_pool_on_origin_thread_->OnIdle();
+  worker_pool_on_origin_thread_->OnIdle(&completed_tasks);
 }
 
 void WorkerPool::Inner::Run() {
@@ -251,29 +408,37 @@ void WorkerPool::Inner::Run() {
   int thread_index = next_thread_index_++;
 
   while (true) {
-    if (pending_tasks_.empty()) {
-      // Exit when shutdown is set and no more tasks are pending.
-      if (shutdown_)
-        break;
-
-      // Schedule an idle callback if requested and not pending.
-      if (!running_task_count_)
-        ScheduleOnIdleWithLockAcquired();
-
-      // Wait for new pending tasks.
-      has_pending_tasks_cv_.Wait();
+    if (ready_to_run_tasks_.empty()) {
+      if (pending_tasks_.empty()) {
+        // Exit when shutdown is set and no more tasks are pending.
+        if (shutdown_)
+          break;
+
+        // Schedule an idle callback if no tasks are running.
+        if (running_tasks_.empty())
+          ScheduleOnIdleWithLockAcquired();
+      }
+
+      // Wait for more tasks.
+      has_ready_to_run_tasks_cv_.Wait();
       continue;
     }
 
-    // Get next task.
-    scoped_ptr<internal::WorkerPoolTask> task = pending_tasks_.take_front();
+    // Take top priority task from |ready_to_run_tasks_|.
+    scoped_refptr<internal::WorkerPoolTask> task(
+        ready_to_run_tasks_.begin()->second);
+    ready_to_run_tasks_.erase(ready_to_run_tasks_.begin());
+
+    // Move task from |pending_tasks_| to |running_tasks_|.
+    DCHECK(pending_tasks_.contains(task));
+    DCHECK(!running_tasks_.contains(task));
+    running_tasks_.set(task, pending_tasks_.take_and_erase(task));
 
-    // Increment |running_task_count_| before starting to run task.
-    running_task_count_++;
+    // There may be more work available, so wake up another worker thread.
+    has_ready_to_run_tasks_cv_.Signal();
 
-    // There may be more work available, so wake up another
-    // worker thread.
-    has_pending_tasks_cv_.Signal();
+    // Call WillRun() before releasing |lock_| and running task.
+    task->WillRun();
 
     {
       base::AutoUnlock unlock(lock_);
@@ -281,15 +446,95 @@ void WorkerPool::Inner::Run() {
       task->RunOnThread(thread_index);
     }
 
-    completed_tasks_.push_back(task.Pass());
+    // This will mark task as finished running.
+    task->DidRun();
+
+    // Now iterate over all dependents to check if they are ready to run.
+    scoped_ptr<ScheduledTask> scheduled_task = running_tasks_.take_and_erase(
+        task);
+    if (scheduled_task) {
+      typedef internal::WorkerPoolTask::TaskVector TaskVector;
+      for (TaskVector::iterator it = scheduled_task->dependents().begin();
+           it != scheduled_task->dependents().end(); ++it) {
+        internal::WorkerPoolTask* dependent = *it;
+        if (!dependent->IsReadyToRun())
+          continue;
+
+        // Task is ready. Add it to |ready_to_run_tasks_|.
+        DCHECK(pending_tasks_.contains(dependent));
+        unsigned priority = pending_tasks_.get(dependent)->priority();
+        DCHECK(!ready_to_run_tasks_.count(priority) ||
+               ready_to_run_tasks_[priority] == dependent);
+        ready_to_run_tasks_[priority] = dependent;
+      }
+    }
 
-    // Decrement |running_task_count_| now that we are done running task.
-    running_task_count_--;
+    // Finally add task to |completed_tasks_|.
+    completed_tasks_.push_back(task);
   }
 
   // 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).
-  has_pending_tasks_cv_.Signal();
+  has_ready_to_run_tasks_cv_.Signal();
+}
+
+// BuildScheduledTaskMap() takes a task tree as input and constructs
+// a unique set of tasks with edges between dependencies pointing in
+// the direction of the dependents. Each task is given a unique priority
+// which is currently the same as the DFS traversal order.
+//
+// Input:             Output:
+//
+//       root               task4          Task | Priority (lower is better)
+//     /      \           /       \      -------+---------------------------
+//  task1    task2      task3    task2     root | 4
+//    |        |          |        |      task1 | 2
+//  task3      |        task1      |      task2 | 3
+//    |        |           \      /       task3 | 1
+//  task4    task4           root         task4 | 0
+//
+// The output can be used to efficiently maintain a queue of
+// "ready to run" tasks.
+
+// static
+unsigned WorkerPool::Inner::BuildScheduledTaskMapRecursive(
+    internal::WorkerPoolTask* task,
+    internal::WorkerPoolTask* dependent,
+    unsigned priority,
+    ScheduledTaskMap* scheduled_tasks) {
+  // Skip sub-tree if task has already completed.
+  if (task->HasCompleted())
+    return priority;
+
+  ScheduledTaskMap::iterator scheduled_it = scheduled_tasks->find(task);
+  if (scheduled_it != scheduled_tasks->end()) {
+    DCHECK(dependent);
+    scheduled_it->second->dependents().push_back(dependent);
+    return priority;
+  }
+
+  typedef internal::WorkerPoolTask::TaskVector TaskVector;
+  for (TaskVector::iterator it = task->dependencies().begin();
+       it != task->dependencies().end(); ++it) {
+    internal::WorkerPoolTask* dependency = *it;
+    priority = BuildScheduledTaskMapRecursive(
+        dependency, task, priority, scheduled_tasks);
+  }
+
+  scheduled_tasks->set(task,
+                       make_scoped_ptr(new ScheduledTask(dependent,
+                                                         priority)));
+
+  return priority + 1;
+}
+
+// static
+void WorkerPool::Inner::BuildScheduledTaskMap(
+    internal::WorkerPoolTask* root,
+    ScheduledTaskMap* scheduled_tasks) {
+  const unsigned kBasePriority = 0u;
+  DCHECK(root);
+  BuildScheduledTaskMapRecursive(root, NULL, kBasePriority, scheduled_tasks);
 }
 
 WorkerPool::WorkerPool(size_t num_threads,
@@ -297,83 +542,105 @@ WorkerPool::WorkerPool(size_t num_threads,
                        const std::string& thread_name_prefix)
     : client_(NULL),
       origin_loop_(base::MessageLoopProxy::current()),
-      weak_ptr_factory_(this),
       check_for_completed_tasks_delay_(check_for_completed_tasks_delay),
       check_for_completed_tasks_pending_(false),
+      in_dispatch_completion_callbacks_(false),
       inner_(make_scoped_ptr(new Inner(this,
                                        num_threads,
                                        thread_name_prefix))) {
 }
 
 WorkerPool::~WorkerPool() {
-  // Cancel all pending callbacks.
-  weak_ptr_factory_.InvalidateWeakPtrs();
-
-  DCHECK_EQ(0u, completed_tasks_.size());
 }
 
 void WorkerPool::Shutdown() {
+  TRACE_EVENT0("cc", "WorkerPool::Shutdown");
+
+  DCHECK(!in_dispatch_completion_callbacks_);
+
   inner_->Shutdown();
-  inner_->CollectCompletedTasks();
-  DispatchCompletionCallbacks();
-}
 
-void WorkerPool::PostTaskAndReply(
-    const Callback& task, const base::Closure& reply) {
-  PostTask(make_scoped_ptr(new WorkerPoolTaskImpl(
-                               task,
-                               reply)).PassAs<internal::WorkerPoolTask>());
+  TaskDeque completed_tasks;
+  inner_->CollectCompletedTasks(&completed_tasks);
+  DispatchCompletionCallbacks(&completed_tasks);
 }
 
-void WorkerPool::OnIdle() {
+void WorkerPool::OnIdle(TaskDeque* completed_tasks) {
   TRACE_EVENT0("cc", "WorkerPool::OnIdle");
 
-  DispatchCompletionCallbacks();
+  DCHECK(!in_dispatch_completion_callbacks_);
+
+  DispatchCompletionCallbacks(completed_tasks);
+
+  // Cancel any pending check for completed tasks.
+  check_for_completed_tasks_callback_.Cancel();
+  check_for_completed_tasks_pending_ = false;
 }
 
 void WorkerPool::ScheduleCheckForCompletedTasks() {
   if (check_for_completed_tasks_pending_)
     return;
+  check_for_completed_tasks_callback_.Reset(
+      base::Bind(&WorkerPool::CheckForCompletedTasks,
+                 base::Unretained(this)));
   origin_loop_->PostDelayedTask(
       FROM_HERE,
-      base::Bind(&WorkerPool::CheckForCompletedTasks,
-                 weak_ptr_factory_.GetWeakPtr()),
+      check_for_completed_tasks_callback_.callback(),
       check_for_completed_tasks_delay_);
   check_for_completed_tasks_pending_ = true;
 }
 
 void WorkerPool::CheckForCompletedTasks() {
   TRACE_EVENT0("cc", "WorkerPool::CheckForCompletedTasks");
-  DCHECK(check_for_completed_tasks_pending_);
+
+  DCHECK(!in_dispatch_completion_callbacks_);
+
+  check_for_completed_tasks_callback_.Cancel();
   check_for_completed_tasks_pending_ = false;
 
+  TaskDeque completed_tasks;
+
   // Schedule another check for completed tasks if not idle.
-  if (!inner_->CollectCompletedTasks())
+  if (!inner_->CollectCompletedTasks(&completed_tasks))
     ScheduleCheckForCompletedTasks();
 
-  DispatchCompletionCallbacks();
+  DispatchCompletionCallbacks(&completed_tasks);
 }
 
-void WorkerPool::DispatchCompletionCallbacks() {
+void WorkerPool::DispatchCompletionCallbacks(TaskDeque* completed_tasks) {
   TRACE_EVENT0("cc", "WorkerPool::DispatchCompletionCallbacks");
 
-  if (completed_tasks_.empty())
+  // Early out when |completed_tasks| is empty to prevent unnecessary
+  // call to DidFinishDispatchingWorkerPoolCompletionCallbacks().
+  if (completed_tasks->empty())
     return;
 
-  while (completed_tasks_.size()) {
-    scoped_ptr<internal::WorkerPoolTask> task = completed_tasks_.take_front();
+  // Worker pool instance is not reentrant while processing completed tasks.
+  in_dispatch_completion_callbacks_ = true;
+
+  while (!completed_tasks->empty()) {
+    scoped_refptr<internal::WorkerPoolTask> task = completed_tasks->front();
+    completed_tasks->pop_front();
     task->DidComplete();
+    task->DispatchCompletionCallback();
   }
 
+  in_dispatch_completion_callbacks_ = false;
+
   DCHECK(client_);
   client_->DidFinishDispatchingWorkerPoolCompletionCallbacks();
 }
 
-void WorkerPool::PostTask(scoped_ptr<internal::WorkerPoolTask> task) {
-  // Schedule check for completed tasks if not pending.
-  ScheduleCheckForCompletedTasks();
+void WorkerPool::ScheduleTasks(internal::WorkerPoolTask* root) {
+  TRACE_EVENT0("cc", "WorkerPool::ScheduleTasks");
+
+  DCHECK(!in_dispatch_completion_callbacks_);
+
+  // Schedule check for completed tasks.
+  if (root)
+    ScheduleCheckForCompletedTasks();
 
-  inner_->PostTask(task.Pass());
+  inner_->ScheduleTasks(root);
 }
 
 }  // namespace cc