lock_node_t tail;

why is 'tail' lock_node_t? tail.next & tail.lock are never used.

IMHO it should be lock_node_t *, pointing to the last item in the list or NULL when the list is empty.

It will make the code below more readable.

    lock_node_t *new_node = malloc (sizeof (lock_node_t));

use calloc or initialize new_node->lock->id to zeroes otherwise, you're expecting that on line 108.