Thread-safety problem of a thread pool implementation in listing 9.8 of C++ Concurrency in Action

Question

I'm reading C++ Concurrency in Action, 2ed. And the writer showed an implementation of thread pool that uses work stealing as follows:

// Listing 9.7 Lock-based queue for work stealing

class work_stealing_queue
{
private:
    typedef function_wrapper data_type;
    std::deque<data_type> the_queue; 
    mutable std::mutex the_mutex;
public:
    work_stealing_queue()
    {}
    work_stealing_queue(const work_stealing_queue& other)=delete;
    work_stealing_queue& operator=(const work_stealing_queue& other)=delete;
    void push(data_type data) 
    {
        std::lock_guard<std::mutex> lock(the_mutex);
        the_queue.push_front(std::move(data));
    }
    bool empty() const
    {
        std::lock_guard<std::mutex> lock(the_mutex);
        return the_queue.empty();
    }
    bool try_pop(data_type& res) 
    {
        std::lock_guard<std::mutex> lock(the_mutex);
        if(the_queue.empty())
        {
            return false;
        }
        res=std::move(the_queue.front());
        the_queue.pop_front();
        return true;
    }
    bool try_steal(data_type& res) 
    {
        std::lock_guard<std::mutex> lock(the_mutex);
        if(the_queue.empty())
        {
            return false;
        }
        res=std::move(the_queue.back());
        the_queue.pop_back();
        return true;
    }
};

// Listing 9.8 A thread pool that uses work stealing

class thread_pool
{
    typedef function_wrapper task_type;
    std::atomic_bool done;
    threadsafe_queue<task_type> pool_work_queue;
    std::vector<std::unique_ptr<work_stealing_queue>> queues; 
    std::vector<std::thread> threads;
    join_threads joiner;
    static thread_local work_stealing_queue* local_work_queue; 
    static thread_local unsigned my_index;
    void worker_thread(unsigned my_index_)
    {
        my_index=my_index_;
        local_work_queue=queues[my_index].get(); 
        while(!done)
        {
            run_pending_task();
        }
    }
    bool pop_task_from_local_queue(task_type& task)
    {
        return local_work_queue && local_work_queue->try_pop(task);
    }
    bool pop_task_from_pool_queue(task_type& task)
    {
        return pool_work_queue.try_pop(task);
    }
    bool pop_task_from_other_thread_queue(task_type& task) 
    {
        for(unsigned i=0;i<queues.size();++i)
        {
            unsigned const index=(my_index+i+1)%queues.size(); 
            if(queues[index]->try_steal(task))
            {
                return true;
            }
        }
        return false;
    }
public:
    thread_pool():
        done(false),joiner(threads)
    {
        unsigned const thread_count=std::thread::hardware_concurrency();
        try
        {
            for(unsigned i=0;i<thread_count;++i)
            {
                queues.push_back(std::unique_ptr<work_stealing_queue>( 
                new work_stealing_queue));
            }
            for(unsigned i=0;i<thread_count;++i)
            {
                threads.push_back(std::thread(&thread_pool::worker_thread,this,i));
            }
        }
        catch(...)
        {
            done=true;
            throw;
        }
    }
    ~thread_pool()
    {
        done=true;
    }
    template<typename FunctionType>
    std::future<typename std::result_of<FunctionType()>::type> submit(
    FunctionType f)
    {
        typedef typename std::result_of<FunctionType()>::type result_type;
        std::packaged_task<result_type()> task(f);
        std::future<result_type> res(task.get_future());
        if(local_work_queue)
        {
            local_work_queue->push(std::move(task));
        }
        else
        {
            pool_work_queue.push(std::move(task));
        }
        return res;
    }
    void run_pending_task()
    {
        task_type task;
        if(pop_task_from_local_queue(task) || 
            pop_task_from_pool_queue(task) || 
            pop_task_from_other_thread_queue(task)) 
        {
            task();
        }
        else
        {
            std::this_thread::yield();
        }
    }
};

Here comes my question: In the constructor of thread_pool, he firstly constructed all the work_stealing_queue and then all the worker threads. And when the execution of a worker thread reaches run_pending_task, it will try to access the thread_pool::queues member varaiable. Is it possible that, for the reason of reordering, the construction of the elements in thread_pool::queues are still not finished before the access from the worker thread? If not, how is the order ensured? I could not find any synchronize-with relationship between these events.

Would you kindly explain the thread safety problem described above?

Thanks for your reading!

Answer 1

The completion of the std::thread constructor call synchronizes-with the start of the invocation of the thread function. (see [thread.thread.constr]/6)

Therefore it is guaranteed that worker_thread will see the state of queues after the loop in which its elements are constructed and there is no data race.