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

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Message-ID: <20080402105539.GA5610@linux.vnet.ibm.com>
Date:	Wed, 2 Apr 2008 03:55:39 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Jens Axboe <jens.axboe@...cle.com>,
	Pekka J Enberg <penberg@...helsinki.fi>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	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, Apr 02, 2008 at 09:28:46AM +0200, Ingo Molnar wrote:
> 
> * Jens Axboe <jens.axboe@...cle.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
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