1. No, use() cannot be called with 0 in this setup. Given the setup,
memory operations take effect in program order (sequential consistency).
The only way for p2() to exit the while (!ready){} loop is to read
ready=1, which implies (by sequential consistency) that p2() will read
data as 2000.

2. an idiomatic way to do this in C is:

    int count_nodes(struct list_node* head)
    {
        int count;
        for (count = 0; head; head = head->next, ++count) {}
        return count;
    }

one could also do:

    int count_nodes(struct list_node* head)
    {
        int count = 0;
        while (head) {
            count++;
            head = head->next;
        }
        return count;
    }


3. b, c