ACCESS_ONCE usage inside llist_add_batch function

[Cihangir Akturk]

Reading the lib/llist.c file in the kernel sources, I came across
the llist_add_bach function defined like this;

bool llist_add_batch(struct llist_node *new_first, struct llist_node *new_last,
                     struct llist_head *head)
{
        struct llist_node *first;

        do {
                new_last->next = first = ACCESS_ONCE(head->first);
        } while (cmpxchg(&head->first, first, new_first) != first);

        return !first;
}

One thing bugging my mind is the ACCESS_ONCE macro. Is it really
needed here ? I mean I would write this function with ACCES_ONCE
moved outside the loop like as follows;

bool llist_add_batch(struct llist_node *new_first, struct llist_node *new_last,
                     struct llist_head *head)
{
        struct llist_node *first, *old;

        old = ACCESS_ONCE(head->first);
        for (;;) {
                first = old;
                new_last->next = old;
                old = cmpxchg(&head->first, first, new_first);
                if (old == first)
                        break;
        }

        return !first;
}

I think that it may be faster to just use the return value of cmpxchg.
But I am not sure about this.

Is my understanding correct ?

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