On Fri, Jun 22, 2012 at 9:22 PM, Joonsoo Kim <js1304@xxxxxxxxx> 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@xxxxxxxxx>
>
> 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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