Re: mutex example

<friend.blah@googlemail.com> wrote in message
news:2b6cbb14-779a-447c-81c6-7c68272fa8a9@d77g2000hsb.googlegroups.com...


Sure... Here is some pseudo-code:
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class mutex {
  enum constant {
    UNLOCKED = 0,
    LOCKED = 1,
    CONTENTION = 2
  };

  atomic_word m_state;
  event m_waitset;

public:
  mutex() : m_state(UNLOCKED) {}

  void lock() {
    if (ATOMIC_SWAP(&m_state, LOCKED)) {
      while (ATOMIC_SWAP(&m_state, CONTENTION)) {
        m_waitset.wait();
      }
    }
    MEMBAR #StoreLoad | #StoreStore;
  }

  void unlock() {
    MEMBAR #LoadStore | #StoreStore;
    if (ATOMIC_SWAP(&m_state, UNLOCKED) == CONTENTION) {
      m_waitset.set();
    }
  }
};
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Do you need mutual exclusion amongst the three threads or not? If you do,
then only a single thread will ever be executing code within the
critical-section at any one time. If not, then you could use a barrier
synchronization object at the end of the function. Basically, something like
Pascal suggested. Except, POSIX has a barrier already, you don't need to
implement one from mutexs and conditions. Something like; modulo any typos
with error checking omitted:

___________________________________________________________________
class barrier {
  pthread_barrier_t m_waitset;

public:
  barrier(unsigned count) {
    pthread_barrier_init(&m_waitset, NULL, count);
  }

  ~barrier() {
    pthread_barrier_destroy(&m_waitset);
  }

  void wait() {
    pthread_barrier_wait(&m_waitset);
  }
};
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You can use it like:
___________________________________________________________________
static barrier* g_barrier = NULL;

int main() {
  barrier m_barrier(3);
  g_barrier = &m_barrier;
  {
    spawn_threads();
    join_threads();
  }
  return 0;
}

void Your_Function_For_The_Three_Threads(...) {
  [...];
  g_barrier->wait();
}

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No problem.