Writing a (spinning) thread barrier using c++11 atomics

Question

I\'m trying to familiarize myself with c++11 atomics, so I tried writing a barrier class for threads (before someone complains about not using existing classes: this is more

Answer 1

It looks needlessly complicated. Try this simpler version (well, I haven't tested it, I just meditated on it:))) :

#include <atomic>

class spinning_barrier
{
public:
    spinning_barrier (unsigned int n) : n_ (n), nwait_ (0), step_(0) {}

    bool wait ()
    {
        unsigned int step = step_.load ();

        if (nwait_.fetch_add (1) == n_ - 1)
        {
            /* OK, last thread to come.  */
            nwait_.store (0); // XXX: maybe can use relaxed ordering here ??
            step_.fetch_add (1);
            return true;
        }
        else
        {
            /* Run in circles and scream like a little girl.  */
            while (step_.load () == step)
                ;
            return false;
        }
    }

protected:
    /* Number of synchronized threads. */
    const unsigned int n_;

    /* Number of threads currently spinning.  */
    std::atomic<unsigned int> nwait_;

    /* Number of barrier syncronizations completed so far, 
     * it's OK to wrap.  */
    std::atomic<unsigned int> step_;
};

EDIT: @Grizzy, I can't find any errors in your first (C++11) version and I've also run it for like a hundred million syncs with two threads and it completes. I've run it on a dual-socket/quad-core GNU/Linux machine though, so I'm rather inclined to suspect your option 3. - the library (or rather, its port to win32) is not mature enough.