I am trying to parallelize a modified BFS but it’s segfaulting. My idea is that it is segafaulting due to the else if section as some threads might be unwantedly accessing it. I cannot figure out any other way to fix it. Can someone suggest any changes?

This is the source for original BFS:
https://p.ip.fi/LTLr

This is another reference:
https://github.com/cmu15418/assignment3/blob/master/bfs/bfs.cpp

void make_bfs_step(int u, int n, vector<int> &frontier, vector<int> &new_frontier, vector<int> &distances, vvi &backedge, vvi &adj, vvi &tree)
{
    // vector<int> frontier, new_frontier;
    int local_count = 0;
    int count = frontier.size();
#pragma omp parallel num_threads(NUM_THREADS)
    // shared(new_frontier, par, vis, tree, frontier)
    {
        vector<int> local_frontier;
        int id = omp_get_thread_num();
        int nthrds = omp_get_num_threads();
        for (int i = id; i < frontier.size(); i += nthrds)
        {
            int node = frontier[i];
            // std::cout << id <<  " here " << node << endl;
            for (auto v : adj[node])
            {
                // cout << v << " " << distances[v] <<  endl;
                if (__sync_bool_compare_and_swap(&distances[v], NOT_VISITED_MARKER, distances[node] + 1))
                {
                    vis[v] = 1;
                    par[v] = node;
                    tree[node].push_back(v);
                    local_frontier.push_back(v);
                }
                else if (par[node] != v)
                    backedge[node].push_back(v);
            }
        }
#pragma omp critical
        {

            new_frontier.insert(new_frontier.end(), local_frontier.begin(), local_frontier.end());
        }
    }
    // cout << "n" << endl;
}

int make_tree_bfs_par(int u, vvi &backedge, vvi &adj, vvi &tree)
{

    vector<int> frontier, new_frontier, distances(n);

    for (int i = 0; i < n; i++)
        distances[i] = NOT_VISITED_MARKER;

    frontier.push_back(u);

    // set the root distance with 0
    distances[u] = 0;

    // just like pop the queue
    while (frontier.size() != 0)
    {

        new_frontier.clear();
        make_bfs_step(u, n, frontier, new_frontier, distances, backedge, adj, tree);

        swap(frontier, new_frontier);
    }
    return n;
}