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Date: Wed, 2 Apr 2008 03:55:39 -0700
From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: Ingo Molnar <mingo@elte.hu>
Cc: Jens Axboe <jens.axboe@oracle.com>,
       Pekka J Enberg <penberg@cs.helsinki.fi>,
       Peter Zijlstra <a.p.zijlstra@chello.nl>,
       Vegard Nossum <vegard.nossum@gmail.com>,
       Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: kmemcheck caught read from freed memory (cfq_free_io_context)
Message-ID: <20080402105539.GA5610@linux.vnet.ibm.com>
Reply-To: paulmck@linux.vnet.ibm.com
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On Wed, Apr 02, 2008 at 09:28:46AM +0200, Ingo Molnar wrote:
> 
> * Jens Axboe <jens.axboe@oracle.com> wrote:
> 
> > On Wed, Apr 02 2008, Pekka J Enberg wrote:
> > > On Wed, 2 Apr 2008, Jens Axboe wrote:
> > > > Good catch, I wonder why it didn't complain in my testing. I've added a
> > > > patch to fix that, please see it here:
> > > 
> > > You probably don't have kmemcheck in your kernel ;-)
> > 
> > Ehm no, you are right :)
> 
> ... and you can get kmemcheck by testing on x86.git/latest:
> 
>   http://people.redhat.com/mingo/x86.git/README
> 
> ;-)

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().

							Thanx, Paul