static int route4_change(...)
{
    ...
    f = kzalloc(sizeof(struct route4_filter), GFP_KERNEL);      [0]
    ...
    // if there exists a filter with the same handler, copy some information
    if (fold) {                                                 [1]
        f->id = fold->id;
        f->iif = fold->iif;
        f->res = fold->res;
        f->handle = fold->handle;

        f->tp = fold->tp;
        f->bkt = fold->bkt;
        new = false;
    }
    
    // initialize the new filter
    err = route4_set_parms(net, tp, base, f, handle, head, tb,  [2]
                   tca[TCA_RATE], new, flags, extack);
    if (err < 0)
        goto errout;

    // insert the new filter to the list
    h = from_hash(f->handle >> 16);                             [3]
    fp = &f->bkt->ht[h];
    for (pfp = rtnl_dereference(*fp);
         (f1 = rtnl_dereference(*fp)) != NULL;
         fp = &f1->next)
        if (f->handle < f1->handle)
            break;

    tcf_block_netif_keep_dst(tp->chain->block);
    rcu_assign_pointer(f->next, f1);
    rcu_assign_pointer(*fp, f);
    
    // remove fold filter from the list if fold exists
    if (fold && fold->handle && f->handle != fold->handle) {    [4]
        th = to_hash(fold->handle);
        h = from_hash(fold->handle >> 16);
        b = rtnl_dereference(head->table[th]);
        if (b) {
            fp = &b->ht[h];
            for (pfp = rtnl_dereference(*fp); pfp;
                 fp = &pfp->next, pfp = rtnl_dereference(*fp)) {
                if (pfp == fold) {
                    rcu_assign_pointer(*fp, fold->next);  [5]// remove the old from the linked list
                    break;
                }
            }
        }
    }
...
    // free the fold filter if it exists                         [6]
    if (fold) {
        tcf_unbind_filter(tp, &fold->res);
        tcf_exts_get_net(&fold->exts);
        tcf_queue_work(&fold->rwork, route4_delete_filter_work);
    }