ResourceLoader* ResourceDispatcherHostImpl::GetLoader(
    const GlobalRequestID& id) const {
  DCHECK_CURRENTLY_ON(BrowserThread::IO);

  LoaderMap::const_iterator i = pending_loaders_.find(id);
  if (i == pending_loaders_.end())
    return NULL;

  return i->second.get();
}

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.