linux-kernel - Re: [PATCH 3/3] slub: release a lock if freeing object with a lock is failed in __slab

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Date:	Thu, 5 Jul 2012 09:26:50 -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 3/3] slub: release a lock if freeing object with a lock
 is failed in __slab_free()

On Sat, 23 Jun 2012, Joonsoo Kim 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.

A free cannot freeze the slab without taking the lock. The taken lock
makes sure that the thread that first enters slab_free() will be able to
hold back the thread that wants to freeze the slab.

> 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.

acquire_slab() also requires lock acquisition and would be held of by
slab_free holding the lock.

> This also make logic somehow simple that 'was_frozen with a lock' case
> is never occured. Remove it.

That is actually interesting and would be a good optimization.

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