QtConcurrent: why releaseThread and reserveThread cause deadlock?

Question

In Qt 4.7 Reference for QThreadPool, we find:

void QThreadPool::releaseThread()

Releases a thread previously reserved by a call to reserveThread().

Note: Calling this function without previously reserving a thread temporarily increases maxThreadCount(). This is useful when a thread goes to sleep waiting for more work, allowing other threads to continue. Be sure to call reserveThread() when done waiting, so that the thread pool can correctly maintain the activeThreadCount().

See also reserveThread().

---

void QThreadPool::reserveThread()

Reserves one thread, disregarding activeThreadCount() and maxThreadCount().

Once you are done with the thread, call releaseThread() to allow it to be reused.

Note: This function will always increase the number of active threads. This means that by using this function, it is possible for activeThreadCount() to return a value greater than maxThreadCount().

See also releaseThread().

I want to use releaseThread() to make it possible to use nested concurrent map, but in the following code, it hangs in waitForFinished():

#include <QApplication>
#include <QMainWindow>
#include <QtConcurrentMap>
#include <QtConcurrentRun>
#include <QFuture>
#include <QThreadPool>
#include <QtTest/QTest>
#include <QFutureSynchronizer>

struct Task2 { // only calculation
    typedef void result_type;
    void operator()(int count) {
        int k = 0;
        for (int i = 0; i < count * 10; ++i) {
            for (int j = 0; j < count * 10; ++j) {
                k++;
            }
        }
        assert(k >= 0);
    }
};

struct Task1 { // will launch some other concurrent map
    typedef void result_type;
    void operator()(int count) {

        QVector<int> vec;
        for (int i = 0; i < 5; ++i) {
            vec.push_back(i+count);
        }
        Task2 task;

        QFuture<void> f = QtConcurrent::map(vec.begin(), vec.end(), task);
        {
            // with out releaseThread before wait, it will hang directly
            QThreadPool::globalInstance()->releaseThread();
            f.waitForFinished(); // BUG: may hang there
            QThreadPool::globalInstance()->reserveThread();
        }
    }
};


int main() {
    QThreadPool* gtpool = QThreadPool::globalInstance();
    gtpool->setExpiryTimeout(50);
    int count = 0;
    for (;;) {
        QVector<int> vec;
        for (int i = 0; i < 40 ; i++) {
            vec.push_back(i);
        }
        // launch a task with nested map
        Task1 task; // Task1 will have nested concurrent map
        QFuture<void> f = QtConcurrent::map(vec.begin(), vec.end(),task);

        f.waitForFinished(); // BUG: may hang there

        count++;

        // waiting most of thread in thread pool expire
        while (QThreadPool::globalInstance()->activeThreadCount() > 0) {
            QTest::qSleep(50);
        }

        // launch a task only calculation
        Task2 task2;
        QFuture<void> f2 = QtConcurrent::map(vec.begin(), vec.end(), task2);

        f2.waitForFinished(); // BUG: may hang there

        qDebug() << count;
    }
    return 0;
}

This code will not run forever; it will hang in after many loops (1~10000), with all threads waiting for condition variable.

My questions are:

1. Why does it hang?
2. Can I fix it and keep the nested concurrent map?

dev env:

Linux version 2.6.32-696.18.7.el6.x86_64; Qt4.7.4; GCC 3.4.5

Windows 7; Qt4.7.4; mingw 4.4.0

Answer 1

The program hangs because of the race condition in QThreadPool when you try to deal with expiryTimeout. Here is the analysis in detail :

The problem in QThreadPool - source

When starting a task, QThreadPool did something along the lines of:

QMutexLocker locker(&mutex);

taskQueue.append(task); // Place the task on the task queue
if (waitingThreads > 0) {
   // there are already running idle thread. They are waiting on the 'runnableReady' 
   // QWaitCondition. Wake one up them up.
   waitingThreads--;
   runnableReady.wakeOne();
} else if (runningThreadCount < maxThreadCount) {
   startNewThread(task);
}

And the the thread's main loop looks like this:

void QThreadPoolThread::run()
{
  QMutexLocker locker(&manager->mutex);
  while (true) {
    /* ... */
    if (manager->taskQueue.isEmpty()) {
      // no pending task, wait for one.
      bool expired = !manager->runnableReady.wait(locker.mutex(), 
                                                  manager->expiryTimeout);
      if (expired) {
        manager->runningThreadCount--;
        return;
      } else {
        continue;
      }
    }
    QRunnable *r = manager->taskQueue.takeFirst();
    // run the task
    locker.unlock();
    r->run();
    locker.relock();
  }
}

The idea is that the thread will wait for a given amount of second for a task, but if no task was added in a given amount of time, the thread expires and is terminated. The problem here is that we rely on the return value of runnableReady. If there is a task that is scheduled at exactly the same time as the thread expires, then the thread will see false and will expire. But the main thread will not restart any other thread. That might let the application hang as the task will never be run.

The quick workaround is to use a long expiryTime (30000 by default) and remove the while loop that waits for the threads expired.

Here is the main function modified, the program runs smoothly in Windows 7, 4 threads used by default :

int main() {
    QThreadPool* gtpool = QThreadPool::globalInstance();
    //gtpool->setExpiryTimeout(50); <-- don't set the expiry Timeout, use the default one.
    qDebug() << gtpool->maxThreadCount();

    int count = 0;
    for (;;) {

        QVector<int> vec;
        for (int i = 0; i < 40 ; i++) {
            vec.push_back(i);
        }
        // launch a task with nested map
        Task1 task; // Task1 will have nested concurrent map
        QFuture<void> f = QtConcurrent::map(vec.begin(), vec.end(),task);

        f.waitForFinished(); // BUG: may hang there

        count++;

        /*
        // waiting most of thread in thread pool expire
        while (QThreadPool::globalInstance()->activeThreadCount() > 0)
        {
            QTest::qSleep(50);
        }
        */

        // launch a task only calculation
        Task2 task2;
        QFuture<void> f2 = QtConcurrent::map(vec.begin(), vec.end(), task2);

        f2.waitForFinished(); // BUG: may hang there

        qDebug() << count ;
    }
    return 0;
}