Re: [RFC][PATCH 6/7] freelist: Lock less freelist

Oleg Nesterov
2020-08-28T14:47:19Z

On 08/27, Peter Zijlstra wrote:
>
> 1 file changed, 129 insertions(+)

129 lines! And I spent more than 2 hours trying to understand these
129 lines ;) looks correct...

However, I still can't understand the usage of _acquire/release ops
in this code.

> +static inline void __freelist_add(struct freelist_node *node, struct freelist_head *list)
> +{
> + /*
> + * Since the refcount is zero, and nobody can increase it once it's
> + * zero (except us, and we run only one copy of this method per node at
> + * a time, i.e. the single thread case), then we know we can safely
> + * change the next pointer of the node; however, once the refcount is
> + * back above zero, then other threads could increase it (happens under
> + * heavy contention, when the refcount goes to zero in between a load
> + * and a refcount increment of a node in try_get, then back up to
> + * something non-zero, then the refcount increment is done by the other
> + * thread) -- so if the CAS to add the node to the actual list fails,
> + * decrese the refcount and leave the add operation to the next thread
> + * who puts the refcount back to zero (which could be us, hence the
> + * loop).
> + */
> + struct freelist_node *head = READ_ONCE(list->head);
> +
> + for (;;) {
> + WRITE_ONCE(node->next, head);
> + atomic_set_release(&node->refs, 1);
> +
> + if (!try_cmpxchg_release(&list->head, &head, node)) {

OK, these 2 _release above look understandable, they pair with
atomic_try_cmpxchg_acquire/try_cmpxchg_acquire in freelist_try_get().

> + /*
> + * Hmm, the add failed, but we can only try again when
> + * the refcount goes back to zero.
> + */
> + if (atomic_fetch_add_release(REFS_ON_FREELIST - 1, &node->refs) == 1)
> + continue;

Do we really need _release ? Why can't atomic_fetch_add_relaxed() work?

> +static inline struct freelist_node *freelist_try_get(struct freelist_head *list)
> +{
> + struct freelist_node *prev, *next, *head = smp_load_acquire(&list->head);
> + unsigned int refs;
> +
> + while (head) {
> + prev = head;
> + refs = atomic_read(&head->refs);
> + if ((refs & REFS_MASK) == 0 ||
> + !atomic_try_cmpxchg_acquire(&head->refs, &refs, refs+1)) {
> + head = smp_load_acquire(&list->head);
> + continue;
> + }
> +
> + /*
> + * Good, reference count has been incremented (it wasn't at
> + * zero), which means we can read the next and not worry about
> + * it changing between now and the time we do the CAS.
> + */
> + next = READ_ONCE(head->next);
> + if (try_cmpxchg_acquire(&list->head, &head, next)) {
> + /*
> + * Yay, got the node. This means it was on the list,
> + * which means should-be-on-freelist must be false no
> + * matter the refcount (because nobody else knows it's
> + * been taken off yet, it can't have been put back on).
> + */
> + WARN_ON_ONCE(atomic_read(&head->refs) & REFS_ON_FREELIST);
> +
> + /*
> + * Decrease refcount twice, once for our ref, and once
> + * for the list's ref.
> + */
> + atomic_fetch_add(-2, &head->refs);

Do we the barriers implied by _fetch_? Why can't atomic_sub(2, refs) work?

> + /*
> + * OK, the head must have changed on us, but we still need to decrement
> + * the refcount we increased.
> + */
> + refs = atomic_fetch_add(-1, &prev->refs);

Cosmetic, but why not atomic_fetch_dec() ?

Oleg.