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

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Date:	Wed, 2 Apr 2008 09:59:33 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Vegard Nossum <vegard.nossum@...il.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 02, 2008 at 06:15:52PM +0200, Vegard Nossum wrote:
> 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.

Though this case apparently does not qualify as misuse until such
time as CLONE_IO is implemented.

And doesn't the current check for ->ctor also potentially hide misuse?

> 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! :-))

Glad you liked them!

And Peter's suggested approach would indeed be more accurate.  But I
will still put my patch forward as a stopgap.  ;-)

							Thanx, Paul
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