linux-kernel - Re: kmemcheck caught read from freed memory (cfq_free_io

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Date:	Wed, 02 Apr 2008 13:14:09 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Pekka Enberg <penberg@...helsinki.fi>
Cc:	Jens Axboe <jens.axboe@...cle.com>, paulmck@...ux.vnet.ibm.com,
	Ingo Molnar <mingo@...e.hu>,
	Vegard Nossum <vegard.nossum@...il.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Christoph Lameter <clameter@....com>
Subject: Re: kmemcheck caught read from freed memory (cfq_free_io_context)

On Wed, 2008-04-02 at 14:11 +0300, Pekka Enberg wrote:
> On Wed, 2008-04-02 at 13:07 +0200, Jens Axboe wrote:
> > > Makes sense, and to me Pauls analysis of the code looks totally correct
> > > - there's no bug there, at least related to hlist traversal and
> > > kmem_cache_free(), since we are under rcu_read_lock() and thus hold off
> > > the grace for freeing.
> 
> On Wed, Apr 2, 2008 at 2:08 PM, Peter Zijlstra <a.p.zijlstra@...llo.nl> wrote:
> >  but what holds off the slab allocator re-issueing that same object and
> >  someone else writing other stuff into it?
> 
> Nothing. So you cannot access the object at all after you've called
> kmem_cache_free(). SLAB_RCU or no SLAB_RCU.

Well, you can, but you have to validate you get the object you were
looking for.

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