}
    if (variables[i]->read_only()) {
      VLOG(4) << "Acquiring reader lock for variable "
              << reinterpret_cast<void*>(variable);
      mu->lock_shared();
      variables[i]->set_shared_lock_held();
    } else {
      VLOG(4) << "Acquiring lock for variable "
              << reinterpret_cast<void*>(variable);
      mu->lock();
      variables[i]->set_lock_held();
    }
    prev = mu;
  }