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

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Date:	Tue, 17 Jan 2012 09:12:33 -0600 (CST)
From:	Christoph Lameter <cl@...ux.com>
To:	Eric Dumazet <eric.dumazet@...il.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

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);
--
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