netdev - Re: Hung task when calling clone() due to netfilter/slab

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Date:	Tue, 17 Jan 2012 16:20:30 +0100
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Christoph Lameter <cl@...ux.com>
Cc:	Sasha Levin <levinsasha928@...il.com>,
	Dave Jones <davej@...hat.com>, davem <davem@...emloft.net>,
	Pekka Enberg <penberg@...nel.org>,
	Matt Mackall <mpm@...enic.com>, kaber@...sh.net,
	pablo@...filter.org, linux-kernel <linux-kernel@...r.kernel.org>,
	linux-mm <linux-mm@...ck.org>, netfilter-devel@...r.kernel.org,
	netdev <netdev@...r.kernel.org>
Subject: Re: Hung task when calling clone() due to netfilter/slab

Le mardi 17 janvier 2012 à 09:12 -0600, Christoph Lameter a écrit :
> On Sun, 15 Jan 2012, Eric Dumazet wrote:
> 
> > As soon as the slub_lock is released, another thread can come and find
> > the new kmem_cache.
> 
> Slabs are not looked up by name. A pointer to kmem_cache is passed to slab
> functions and that pointer is returned from kmem_cache_create(). The risk
> is someone traversing the kmem_cach list which is only done from within slub.
> 
> 
> Subject: slub: Do not hold slub_lock when calling sysfs_slab_add()
> 
> sysfs_slab_add() calls various sysfs functions that actually may
> end up in userspace doing all sorts of things.
> 
> Release the slub_lock after adding the kmem_cache structure to the list.
> At that point the address of the kmem_cache is not known so we are
> guaranteed exlusive access to the following modifications to the
> kmem_cache structure.
> 
> If the sysfs_slab_add fails then reacquire the slub_lock to
> remove the kmem_cache structure from the list.
> 
> Signed-off-by: Christoph Lameter <cl@...ux.com>
> 
> ---
>  mm/slub.c |    4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> Index: linux-2.6/mm/slub.c
> ===================================================================
> --- linux-2.6.orig/mm/slub.c	2012-01-17 03:07:11.140010438 -0600
> +++ linux-2.6/mm/slub.c	2012-01-17 03:07:19.532010264 -0600
> @@ -3929,13 +3929,15 @@ struct kmem_cache *kmem_cache_create(con
>  		if (kmem_cache_open(s, n,
>  				size, align, flags, ctor)) {
>  			list_add(&s->list, &slab_caches);
> +			up_write(&slub_lock);
>  			if (sysfs_slab_add(s)) {
> +				down_write(&slub_lock);
>  				list_del(&s->list);
> +				up_write(&slub_lock);
>  				kfree(n);
>  				kfree(s);
>  				goto err;
>  			}
> -			up_write(&slub_lock);
>  			return s;
>  		}
>  		kfree(n);



Buggy patch, since "goto err;" is going to up_write(&slub_lock) again.

Also Christoph, you forgot to add the very much needed :

Reported-by: Sasha Levin <levinsasha928@...il.com>



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