atomic shared pointer about mlib

[P-p-H-d]

Proposition for _set_atomic we store temporary NULL to the atomic_ptr struct to request an exclusive access to the data (this looks like a lock and other operations need to handle NULL) :

void shared_ptr_set_atomic(ptr a, atomic_ptr *ptr)
{
  // Get exclusive access to the data
  p = atomic_load(ptr);
  do {
    if (p == NULL) {
      // TODO: exponential backoff
      p = atomic_load(ptr);
      continue;
    }
  } while (!atomic_compare_exchange_weak(ptr, &p, NULL));
  // p has exclusive access to the pointer
  p->data->cpt ++;
  a->data = p->data;
  atomic_exchange (ptr, p);
}

This prevents using NULL which obliges atomic shared pointer to point to a created object...

Other alternative solution is to use the bit 0 to mark the pointer as being updated, preventing other from using it (TBC only clear):

void shared_ptr_set_atomic(ptr a, atomic_ptr *ptr)
{
  // Get exclusive access to the data
  p = atomic_load(ptr);
  do {
    if ( (p&1) != 0) {
      // TODO: exponential backoff
      p = atomic_load(ptr);
      continue;
    }
  } while (!atomic_compare_exchange_weak(ptr, &p, p|1));
 // Exclusive access (kind of lock).
  p->data->cpt ++;
  a->data = p->data;
  atomic_set (ptr, p);
}

Other implementation seems to have it hard to be lock-free: cf. llvm-mirror/libcxx@5fec82d