allocator is subject to ABA problem

[Lingxi-Li]

For example, allocator::allocate() is currently implemented as

  T* allocate(std::size_t = 1) {
    auto oldhead = head.load(acq);
    do {
      if (!oldhead) throw std::bad_alloc{};
    }
    while (!head.compare_exchange_weak(oldhead, oldhead->next, rlx, acq));
    return std::addressof(oldhead->data);
  }

When head compares equal to oldhead, oldhead->next may already be outdated and invalid.