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

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Date:	Wed, 2 Apr 2008 18:15:52 +0200
From:	"Vegard Nossum" <vegard.nossum@...il.com>
To:	paulmck@...ux.vnet.ibm.com
Cc:	"Pekka Enberg" <penberg@...helsinki.fi>,
	"Ingo Molnar" <mingo@...e.hu>,
	"Jens Axboe" <jens.axboe@...cle.com>,
	"Peter Zijlstra" <a.p.zijlstra@...llo.nl>,
	"Linux Kernel Mailing List" <linux-kernel@...r.kernel.org>
Subject: Re: kmemcheck caught read from freed memory (cfq_free_io_context)

On Wed, Apr 2, 2008 at 6:08 PM, Paul E. McKenney
<paulmck@...ux.vnet.ibm.com> wrote:
> On Wed, Apr 02, 2008 at 02:01:13PM +0300, Pekka Enberg wrote:
>  > 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.
>
>  Would the following be an appropriate fix?  It seems to me to be in
>  the same spirit as the existing check for s->ctor.

In my opinion, no.

It would fix the false positives, but would in fact also hide cases
such as this one with cfq, e.g. the real cases of mis-use.

Peter Zijlstra suggested this:
> It would have to register an call_rcu callback itself in order to mark
> it freed - and handle the race with the object being handed out again.

I will try to look into this -- for now, I need to understand RCU
first (I've seen your LWN articles -- great work! :-))


Kind regards,
Vegard Nossum
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