Synchronizing three threads with Condition Variabl

问题:

I have three threads in my application, the first thread needs to wait for a data to be ready from the two other threads. The two threads are preparing the data concurrently. In order to do that I am using condition variable in C++ as following:

boost::mutex mut;       
boost::condition_variable cond;     

Thread1:

    bool check_data_received()
    {
         return (data1_received && data2_received);
    }

    // Wait until socket data has arrived
    boost::unique_lock<boost::mutex> lock(mut);
    if (!cond.timed_wait(lock, boost::posix_time::milliseconds(200),
        boost::bind(&check_data_received)))
    {

    }

Thread2:

    {
        boost::lock_guard<boost::mutex> lock(mut);
        data1_received = true;
    }
    cond.notify_one();

Thread3:

    {
        boost::lock_guard<boost::mutex> lock(mut);
        data2_received = true;
    }
    cond.notify_one();

So my question is it correct to do that, or is there any more efficient way? I am looking for the most optimized way to do the waiting.

回答1:

It looks like you want a semaphore here, so you can wait for two "resources" to be "taken".

For now, just replace the mutual exclusion with an atomic. you can still use a cv to signal the waiter:

#include <boost/thread.hpp>

boost::mutex mut;       
boost::condition_variable cond;     

boost::atomic_bool data1_received(false);
boost::atomic_bool data2_received(false);

bool check_data_received()
{
    return (data1_received && data2_received);
}

void thread1()
{
    // Wait until socket data has arrived
    boost::unique_lock<boost::mutex> lock(mut);
    while (!cond.timed_wait(lock, boost::posix_time::milliseconds(200),
        boost::bind(&check_data_received)))
    {
        std::cout << "." << std::flush;
    }
}

void thread2()
{
    boost::this_thread::sleep_for(boost::chrono::milliseconds(rand() % 4000));
    data1_received = true;
    cond.notify_one();
}

void thread3()
{
    boost::this_thread::sleep_for(boost::chrono::milliseconds(rand() % 4000));
    data2_received = true;
    cond.notify_one();
}

int main()
{
    boost::thread_group g;
    g.create_thread(thread1);
    g.create_thread(thread2);
    g.create_thread(thread3);

    g.join_all();
}

Note:

• warning - it's essential that you know only the waiter is waiting on the cv, otherwise you need notify_all() instead of notify_one().
• It is not important that the waiter is already waiting before the workers signal their completion, because the predicated timed_wait checks the predicate before blocking.
• Because this sample uses atomics and predicated wait, it's not actually critical to signal the cv under the mutex. However, thread checkers will (rightly) complain about this (I think) because it's impossible for them to check proper synchronization unless you add the locking.