Re: [PATCH 3/3] slub: release a lock if freeing object with a lock is failed in __slab_free()

[Pekka Enberg <penberg@kernel.org>]

On Fri, Jun 22, 2012 at 9:22 PM, Joonsoo Kim <js1304@gmail.com> wrote:
> In some case of __slab_free(), we need a lock for manipulating partial list.
> If freeing object with a lock is failed, a lock doesn't needed anymore
> for some reasons.
>
> Case 1. prior is NULL, kmem_cache_debug(s) is true
>
> In this case, another free is occured before our free is succeed.
> When slab is full(prior is NULL), only possible operation is slab_free().
> So in this case, we guess another free is occured.
> It may make a slab frozen, so lock is not needed anymore.
>
> Case 2. inuse is NULL
>
> In this case, acquire_slab() is occured before out free is succeed.
> We have a last object for slab, so other operation for this slab is
> not possible except acquire_slab().
> Acquire_slab() makes a slab frozen, so lock is not needed anymore.
>
> Above two reason explain why we don't need a lock
> when freeing object with a lock is failed.
>
> So, when cmpxchg_double_slab() is failed, releasing a lock is more suitable.
> This may reduce lock contention.
>
> This also make logic somehow simple that 'was_frozen with a lock' case
> is never occured. Remove it.
>
> Signed-off-by: Joonsoo Kim <js1304@gmail.com>
>
> diff --git a/mm/slub.c b/mm/slub.c
> index 531d8ed..3e0b9db 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -2438,7 +2438,6 @@ static void __slab_free(struct kmem_cache *s, struct page *page,
>         void *prior;
>         void **object = (void *)x;
>         int was_frozen;
> -       int inuse;
>         struct page new;
>         unsigned long counters;
>         struct kmem_cache_node *n = NULL;
> @@ -2450,13 +2449,17 @@ static void __slab_free(struct kmem_cache *s, struct page *page,
>                 return;
>
>         do {
> +               if (unlikely(n)) {
> +                       spin_unlock_irqrestore(&n->list_lock, flags);
> +                       n = NULL;
> +               }
>                 prior = page->freelist;
>                 counters = page->counters;
>                 set_freepointer(s, object, prior);
>                 new.counters = counters;
>                 was_frozen = new.frozen;
>                 new.inuse--;
> -               if ((!new.inuse || !prior) && !was_frozen && !n) {
> +               if ((!new.inuse || !prior) && !was_frozen) {
>
>                         if (!kmem_cache_debug(s) && !prior)
>
> @@ -2481,7 +2484,6 @@ static void __slab_free(struct kmem_cache *s, struct page *page,
>
>                         }
>                 }
> -               inuse = new.inuse;
>
>         } while (!cmpxchg_double_slab(s, page,
>                 prior, counters,
> @@ -2507,25 +2509,17 @@ static void __slab_free(struct kmem_cache *s, struct page *page,
>                  return;
>          }
>
> +       if (unlikely(!new.inuse && n->nr_partial > s->min_partial))
> +               goto slab_empty;
> +
>         /*
> -        * was_frozen may have been set after we acquired the list_lock in
> -        * an earlier loop. So we need to check it here again.
> +        * Objects left in the slab. If it was not on the partial list before
> +        * then add it.
>          */
> -       if (was_frozen)
> -               stat(s, FREE_FROZEN);
> -       else {
> -               if (unlikely(!inuse && n->nr_partial > s->min_partial))
> -                        goto slab_empty;
> -
> -               /*
> -                * Objects left in the slab. If it was not on the partial list before
> -                * then add it.
> -                */
> -               if (unlikely(!prior)) {
> -                       remove_full(s, page);
> -                       add_partial(n, page, DEACTIVATE_TO_TAIL);
> -                       stat(s, FREE_ADD_PARTIAL);
> -               }
> +       if (kmem_cache_debug(s) && unlikely(!prior)) {
> +               remove_full(s, page);
> +               add_partial(n, page, DEACTIVATE_TO_TAIL);
> +               stat(s, FREE_ADD_PARTIAL);
>         }
>         spin_unlock_irqrestore(&n->list_lock, flags);
>         return;

I'm confused. Does this fix a bug or is it an optimization?

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