Author: James CE Johnson

ACE Tutorial 017
Using the ACE_Barrier synch object

The Barrier implementation is quite simple. The threads() mutator took a couple of tries to get right. In particular, be sure you know when to apply the _wait paramter and when not to! In fact, the requirement that only the "owning" thread can change the thread count is rather limiting. A more appropriate solution would allow any thread to safely change the count but that would require more complex locking that is just a bit more than what I wanted to present here.

    // page04.html,v 1.8 1999/04/03 21:56:32 jcej Exp

    #include "Barrier_i.h"

    /* Initialize the threads_ count to zero and the barrier_ pointer to a
       safe value.  At the same time, we remember the thread that created
       us so that we can allow it to change the thread count.
    */
    Barrier::Barrier(void)
       : threads_(0)
        ,barrier_(0)
        ,new_barrier_(0)
    {
        owner_ = ACE_OS::thr_self();
    }

    /* Ensure that barrier_ get's deleted so that we don't have a memory leak.
     */
    Barrier::~Barrier(void)
    {
        delete barrier_;
    }

    void Barrier::owner( ACE_thread_t _owner )
    {
        owner_ = _owner;
    }

    // Report on the number of threads.
    u_int Barrier::threads(void)
    {
        return threads_.value();
    }

    /* Allow the owning thread to (re)set the number of threads.
       make_barrier() is called because it will wait() if we were already
       configured.  Typical usage would be for the worker threads to
       wait() while the primary (eg -- owner) thread adjusts the thread
       count.

       For instance:
           In the worker threads:
               if( myBarrier.threads() != current_thread_count )
                   myBarrier.wait();

           In the primary thread:
               if( myBarrier.threads() != current_thread_count )
                   myBarrier.threads( current_thread_count, 1 );
     */
    int Barrier::threads( u_int _threads, int _wait )
    {
        if( ! ACE_OS::thr_equal(ACE_OS::thr_self(), owner_) )
        {
            return -1;
        }

        threads_ = _threads;

        return make_barrier(_wait);
    }

    /* Wait for all threads to synch if the thread count is valid.  Note
       that barrier_ will be 0 if the threads() mutator has not been
       invoked.
    */
    int Barrier::wait(void)
    {
        if( ! barrier_ )
        {
            return -1;
        }

            // If the threads() mutator has been used, new_barrier_ will
            // point to a new ACE_Barrier instance.  We'll use a
            // traditional double-check here to move that new object into
            // place and cleanup the old one.
        if( new_barrier_ )
        {
                // mutex so that only one thread can do this part.
            ACE_Guard<ACE_Mutex> mutex(barrier_mutex_);

                // We only want the first thread to plug in the new barrier...
            if( new_barrier_ )
            {
                    // out with the old and in with the new.
                delete barrier_;
                barrier_ = new_barrier_;
                new_barrier_ = 0;
            }
        }

        return barrier_->wait();
    }

    /* Wait for all threads to synch.  As each thread passes wait(), it
       will decrement our thread counter.  (That is why we had to make
       threads_ an atomic op.)  When the last thread decrements the
       counter it will also delete the ACE_Barrier & free up a little
       memory.
    */
    int Barrier::done(void)
    {
        if( this->wait() == -1 )
        {
            return -1;
        }

        --threads_;

        if( ! threads_.value() )
        {
            delete barrier_;
            barrier_ = 0;
        }

        return 0;
    }

    /* This will build the actual barrier.  I broke this code out of the
       threads() mutator in case it might be useful elsewhere.
       If a barrier already exists, we will wait for all threads before
       creating a new one.  This trait is what allows the threads mutator
       to be used as shown above.
     */
    int Barrier::make_barrier( int _wait )
    {
            // Ensure we have a valid thread count.
        if( ! threads_.value() )
        {
            return -1;
        }

            // If a barrier already exists, we'll arrange for it to be
            // replaced through the wait() method above.
        if( barrier_ )
        {
                // Create the new barrier that wait() will install for us.
            ACE_NEW_RETURN(new_barrier_,ACE_Barrier(threads_.value()),-1);

                // Wait for our siblings to synch before continuing
            if( _wait )
            {
                barrier_->wait();
            }
        }
        else
        {
                // Create the initial barrier.
            ACE_NEW_RETURN(barrier_,ACE_Barrier(threads_.value()),-1);
        }

        return 0;
    }

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