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

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Date:	Wed, 02 Apr 2008 13:08:56 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Jens Axboe <jens.axboe@...cle.com>
Cc:	Pekka Enberg <penberg@...helsinki.fi>, 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>
Subject: Re: kmemcheck caught read from freed memory (cfq_free_io_context)

On Wed, 2008-04-02 at 13:07 +0200, Jens Axboe wrote:
> On Wed, Apr 02 2008, Pekka Enberg wrote:
> > Hi Paul,
> > 
> > On Wed, Apr 2, 2008 at 1:55 PM, Paul E. McKenney
> > <paulmck@...ux.vnet.ibm.com> wrote:
> > >  I will check this when I get back to some bandwidth -- but in the meantime,
> > >  does kmemcheck special-case SLAB_DESTROY_BY_RCU?  It is legal to access
> > >  newly-freed items in that case, as long as you did rcu_read_lock()
> > >  before gaining a reference to them and don't hold the reference past
> > >  the matching rcu_read_unlock().
> > 
> > No, kmemcheck is work in progress and does not know about
> > SLAB_DESTROY_BY_RCU yet. The reason I asked Vegard to post the warning
> > was because Peter, Vegard, and myself identified this particular
> > warning as a real problem. But yeah, kmemcheck can cause false
> > positives for RCU for now.
> 
> 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.

but what holds off the slab allocator re-issueing that same object and
someone else writing other stuff into it?

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