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Date:	Mon, 19 Mar 2012 18:45:07 +0200
From:	Sasha Levin <levinsasha928@...il.com>
To:	Christoph Lameter <cl@...ux.com>
Cc:	Pablo Neira Ayuso <pablo@...filter.org>, kaber@...sh.net,
	davem@...emloft.net, Dave Jones <davej@...hat.com>,
	netfilter-devel@...r.kernel.org,
	"linux-kernel@...r.kernel.org List" <linux-kernel@...r.kernel.org>,
	netdev@...r.kernel.org
Subject: Re: netfilter: Hung task

On Mon, Mar 19, 2012 at 6:31 PM, Christoph Lameter <cl@...ux.com> wrote:
> On Mon, 19 Mar 2012, Sasha Levin wrote:
>
>> On Mon, Mar 19, 2012 at 3:56 PM, Christoph Lameter <cl@...ux.com> wrote:
>> > This is sually something causing memory corruption. Please enable
>> > debugging to get backtrace that help to debutg this. CONFIG_SLUB_DEBUG_ON
>> > will do the trick or passing "slub_debug" on the kernel command line.
>>
>> The kernel was compiled with SLUB_DEBUG_ON, there's nothing coming out
>> of the slub before that hang message, nor after it.
>
> Ok looking at the backtrace: This is kmem_cache_destroy and not the usual
> failure following a pointer in alloc / free.
>
> netfilter calls kmem_cache_destroy which calls into sysfs functions and
> there the hang occurs.
>
> Did you try to see if lockdep can detect any serialization problems ?
>
> Is kmem_cache_destroy called with any locks held? Interrupts off?
>

lockdep listed all the held locks there.

I've mentioned that it looks very similar to
https://lkml.org/lkml/2012/1/14/45 where the userspace helper tried to
read from the sysfs files, but got into a deadlock since the kernel
side held them before it called the usermode helper.
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