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

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Message-ID: <84144f020804020401j4e5863dcofd16662baa54574@mail.gmail.com>
Date:	Wed, 2 Apr 2008 14:01:13 +0300
From:	"Pekka Enberg" <penberg@...helsinki.fi>
To:	paulmck@...ux.vnet.ibm.com
Cc:	"Ingo Molnar" <mingo@...e.hu>,
	"Jens Axboe" <jens.axboe@...cle.com>,
	"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)

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.

                               Pekka
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