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SubjectRe: 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@linux.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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