void ResourceDispatcherHostImpl::BlockRequestsForRoute(int child_id,
                                                       int route_id) {
  DCHECK_CURRENTLY_ON(BrowserThread::IO);
  GlobalRoutingID key(child_id, route_id);
  DCHECK(blocked_loaders_map_.find(key) == blocked_loaders_map_.end()) <<
      "BlockRequestsForRoute called  multiple time for the same RVH";
  blocked_loaders_map_[key] = make_scoped_ptr(new BlockedLoadersList());
}

The product contains a concurrent code sequence that requires temporary, exclusive access to a shared resource, but a timing window exists in which the shared resource can be modified by another code sequence operating concurrently.