linux-kernel - Re: [PATCH] slub: remove one code path and reduce lock contention in __slab

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Date:	Fri, 27 Jul 2012 15:46:24 -0500 (CDT)
From:	Christoph Lameter <cl@...ux.com>
To:	Joonsoo Kim <js1304@...il.com>
cc:	Pekka Enberg <penberg@...nel.org>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org
Subject: Re: [PATCH] slub: remove one code path and reduce lock contention
 in __slab_free()

On Sat, 28 Jul 2012, Joonsoo Kim wrote:

> Subject and commit log are changed from v1.

That looks a bit better. But the changelog could use more cleanup and
clearer expression.

> @@ -2490,25 +2492,17 @@ 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;
> +

So we can never encounter a empty slab that was frozen before? Really?

Remote frees can decrement inuse again. All objects of a slab frozen on
one cpu could be allocated while the slab is still frozen. The
unfreezing requires slab_alloc to encounter a NULL pointer after all.

A remote processor could obtain a pointer to all these objects and free
them. The code here would cause an unfreeze action. Another alloc on the
first processor would cause a *second* unfreeze action on a page that was
freed.
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