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SubjectRe: Hung task when calling clone() due to netfilter/slab
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Le dimanche 15 janvier 2012 à 14:59 +0200, Sasha Levin a écrit :
> On Sat, 2012-01-14 at 18:10 +0100, Eric Dumazet wrote:
> > Apparently SLUB calls sysfs_slab_add() from kmem_cache_create() while
> > still holding slub_lock.
> >
> > So if the task launched needs to "cat /proc/slabinfo" or anything
> > needing slub_lock, its a deadlock.
>
> I've made the following patch to test it, It doesn't look like it's
> the correct solution, but it verifies that the problem is there (it
> works well with the patch).
>
> ---------------
>
> From cc4874b491b8e5d9d1ea5bf2032413efdbddced8 Mon Sep 17 00:00:00 2001
> From: Sasha Levin <levinsasha928@gmail.com>
> Date: Sun, 15 Jan 2012 14:55:03 +0200
> Subject: [PATCH] slab: Fix hang when creating sysfs entries
>
> This patch fixes the hang which happens when we create a sysfs entry and call
> back to userspace. If the usermode helper tries to do anything which involves
> slub_lock we will hang since slub_lock is already held.
>
> Signed-off-by: Sasha Levin <levinsasha928@gmail.com>
> ---
> mm/slub.c | 2 ++
> 1 files changed, 2 insertions(+), 0 deletions(-)
>
> diff --git a/mm/slub.c b/mm/slub.c
> index 4907563..6948327 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -5283,7 +5283,9 @@ static int sysfs_slab_add(struct kmem_cache *s)
> kobject_put(&s->kobj);
> return err;
> }
> + up_write(&slub_lock);
> kobject_uevent(&s->kobj, KOBJ_ADD);
> + down_write(&slub_lock);
> if (!unmergeable) {
> /* Setup first alias */
> sysfs_slab_alias(s, s->name);
> --
> 1.7.8.3
>
>

Oh well, that cannot be right. Dont send official patches if you already
know "it's not the correct solution", we already know where is the
problem.

It's _never_ right to release a lock for a short time without any
additional checks. [ If it was right, the lock would be not needed ]

As soon as the slub_lock is released, another thread can come and find
the new kmem_cache.

For example, it can destroy it, and your thread is going to access s
while it was already freed.

sysfs_slab_alias(s, s->name); // crash


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