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

[Eric Dumazet]

Le dimanche 15 janvier 2012 Ã 14:59 +0200, Sasha Levin a Ãcrit :
> 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@xxxxxxxxx>
> 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@xxxxxxxxx>
> ---
>  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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