Re: [PATCH -mm v2 5/8] slub: make slab_free non-preemptable

[Vladimir Davydov <vdavydov@parallels.com>]

On Fri, Jun 06, 2014 at 09:46:57AM -0500, Christoph Lameter wrote:
> On Fri, 6 Jun 2014, Vladimir Davydov wrote:
> 
> > This patch makes SLUB's implementation of kmem_cache_free
> > non-preemptable. As a result, synchronize_sched() will work as a barrier
> > against kmem_cache_free's in flight, so that issuing it before cache
> > destruction will protect us against the use-after-free.
> 
> 
> Subject: slub: reenable preemption before the freeing of slabs from slab_free
> 
> I would prefer to call the page allocator with preemption enabled if possible.
> 
> Signed-off-by: Christoph Lameter <cl@linux.com>
> 
> Index: linux/mm/slub.c
> ===================================================================
> --- linux.orig/mm/slub.c	2014-05-29 11:45:32.065859887 -0500
> +++ linux/mm/slub.c	2014-06-06 09:45:12.822480834 -0500
> @@ -1998,6 +1998,7 @@
>  	if (n)
>  		spin_unlock(&n->list_lock);
> 
> +	preempt_enable();

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.

>  	while (discard_page) {
>  		page = discard_page;
>  		discard_page = discard_page->next;
> @@ -2006,6 +2007,7 @@
>  		discard_slab(s, page);

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.

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.

Thanks.

>  		stat(s, FREE_SLAB);
>  	}
> +	preempt_disable();
>  #endif
>  }
>