29/hand-over-hand-locking-list.c has data race when inserting node

Hi there, First, I want to thank you for this amazing repo which helps me a lot.

static void List_Insert(list_t *L, int key) {
  // synchronization not needed
  node_t *new = malloc(sizeof(node_t));
  if (new == NULL)
    handle_error_en(errno, "malloc");
  new->key = key;
  Pthread_mutex_init(&new->lock, NULL);

  // just lock critical section
  pthread_mutex_t *tempLock = &L->head->lock;
  Pthread_mutex_lock(tempLock);
  new->next = L->head;
  L->head = new;
  Pthread_mutex_unlock(tempLock);
}

This snippet of code does not eliminate the race condition when there are more than two threads inserting nodes to the list. Imagine thread_1 and thread_2 try to lock the head of the list and only one thread succeded. The other thread is blocked but holding the same head node as the other thread. After the thread(which successfully locked the head node of the list) inserting a new node into the list, what the blocked thread should do is locking the new head. But it will insert a node as the predecessor of the old head node.

I think we still need a global lock for inserting nodes into the list.

xxyzz / ostep-hw

29/hand-over-hand-locking-list.c has data race when inserting node #8