linux-kernel - Re: [PATCH -mm v2 5/8] slub: make slab

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Message-ID: <alpine.DEB.2.10.1406090850070.22191@gentwo.org>
Date:	Mon, 9 Jun 2014 08:52:24 -0500 (CDT)
From:	Christoph Lameter <cl@...two.org>
To:	Vladimir Davydov <vdavydov@...allels.com>
cc:	akpm@...ux-foundation.org, iamjoonsoo.kim@....com,
	rientjes@...gle.com, penberg@...nel.org, hannes@...xchg.org,
	mhocko@...e.cz, linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH -mm v2 5/8] slub: make slab_free non-preemptable

On Mon, 9 Jun 2014, Vladimir Davydov wrote:

> The whole function (unfreeze_partials) is currently called with irqs
> off, so this is effectively a no-op. I guess we can restore irqs here
> though.

We could move the local_irq_save from put_cpu_partial() into
unfreeze_partials().

> If we just freed the last slab of the cache and then get preempted
> (suppose we restored irqs above), nothing will prevent the cache from
> destruction, which may result in use-after-free below. We need to be
> more cautious if we want to call for page allocator with preemption and
> irqs on.

Hmmm. Ok.
>
> However, I still don't understand what's the point in it. We *already*
> call discard_slab with irqs disabled, which is harder, and it haven't
> caused any problems AFAIK. Moreover, even if we enabled preemption/irqs,
> it wouldn't guarantee that discard_slab would always be called with
> preemption/irqs on, because the whole function - I mean kmem_cache_free
> - can be called with preemption/irqs disabled.
>
> So my point it would only complicate the code.

Ok.

Acked-by: Christoph Lameter <cl@...ux.com>

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