nonrecursive_filter.c

void filter(list_item *item, int (*passes)(list_item *item))
{
    pthread_mutex_lock(&list_lock);
    while (item != NULL)
    {
        if (!passes(item))
            _remove_internal(item);
        item = item->next;
    }
    pthread_mutex_unlock(&list_lock);
}

void remove(list_item *item)
{
    pthread_mutex_lock(&list_lock);
    _remove_internal(item);
    pthread_mutex_unlock(&list_lock);
}

void _remove_internal(list_item *item)
{
    if (item->prev != NULL)
        item->prev->next = item->next;
    if (item->next != NULL)
        item->next->prev = item->prev;
}

recursive_filter.c

void filter(list_item *item, int (*passes)(list_item *item))
{
    pthread_mutex_lock(&list_lock);
    while (item != NULL)
    {
        if (!passes(item))
            remove(item);
        item = item->next;
    }
    pthread_mutex_unlock(&list_lock);
}

void remove(list_item *item)
{
    pthread_mutex_lock(&list_lock);
    if (item->prev != NULL)
        item->prev->next = item->next;
    if (item->next != NULL)
        item->next->prev = item->prev;
    pthread_mutex_unlock(&list_lock);
}