Question

I am trying to create a concurrent queue with Qt’s concurrent threading constructs.

#ifndef CONCURRENTQUEUE_H
#define CONCURRENTQUEUE_H
#include <QMutex>
#include <QWaitCondition>
#include <queue>

template<typename Data>
class ConcurrentQueue
{
private:
    std::queue<Data> the_queue;
    QMutex the_mutex;
    QWaitCondition the_condition_variable;
    bool closed;

public:

    void setClosed(bool state)
    {
        QMutexLocker locker(&the_mutex);
        closed = state;    
    }

    bool getClosed()
    {
        QMutexLocker locker(&the_mutex);
        return closed;    
    }

    void push(Data const& data)
    {
        QMutexLocker locker(&the_mutex);
        the_queue.push(data);
        the_condition_variable.wakeOne();    
    }

    bool empty()
    {
        QMutexLocker locker(&the_mutex);
        return the_queue.empty();    
    }

    bool try_pop(Data& popped_value)
    {
        QMutexLocker locker(&the_mutex);
        if(the_queue.empty())
        {
            return false;
        }
        popped_value = the_queue.front();
        the_queue.pop();
        return true;
    }

    void wait_and_pop(Data& popped_value)
    {
        QMutexLocker locker(&the_mutex);
        while(the_queue.empty())
        {
            the_condition_variable.wait(&the_mutex);
        }
        popped_value = the_queue.front();
        the_queue.pop();
        the_condition_variable.wakeOne();
    }

    //created to allow for a limited queue size
    void wait_and_push(Data const& data, const int max_size)
    {
        QMutexLocker locker(&the_mutex);
        while(the_queue.size() >= max_size)
        {
            the_condition_variable.wait(&the_mutex);
        }
        the_queue.push(data);
        the_condition_variable.wakeOne();
    }


};

#endif // CONCURRENTQUEUE_H

I have my producer thread using the wait_and_push method to push data into the queue, and I have been trying to get my consumer to read from the queue using try_pop

 while(!tiles->empty() || !tiles->getClosed())
{
             if(!tiles->try_pop(tile))
                    continue;
//do stuff with the tile
}

However, this deadlocks sometimes. The producer sets the closed boolean as a flag to the consumer threads that it is finished loading the queue. My consumer only has that as a way to know whether teh queue is being loaded, still in progress, or hasnt been started.

The reason the producer has a “wait_and_push” isntead of using the normal push is because I wanted to be able to make that thread block until some items had been processed to avoid eating up so much memory, and doing unnecessary disk I/O.

Can anyone point me to what is going wrong?

Answer 1

You forgot to add

the_condition_variable.wakeOne();

in try_pop.

If there will be multiple producers / consumers accessing your queue, you should have a separate QWaitCondition for producers and consumers, otherwise wakeOne might not wake the correct thread.

EDIT:

If there will be multiple producers / consumers, then you should have a notFullCondvar, and an notEmptyCondvar.

• The try_pop method wakes the notFullCondvar.
• The wait_and_pop method waits on the notEmptyCondvar, but wakes the notFullCondvar.
• The push method wakes the notEmptyCondvar.
• The wait_and_push method waits on the notFullCondvar, but wakes the notEmptyCondvar.

I hope this makes sense.