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

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Message-ID: <20080402110718.GU12774@kernel.dk>
Date:	Wed, 2 Apr 2008 13:07:19 +0200
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Pekka Enberg <penberg@...helsinki.fi>
Cc:	paulmck@...ux.vnet.ibm.com, Ingo Molnar <mingo@...e.hu>,
	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, Pekka Enberg wrote:
> 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.

Makes sense, and to me Pauls analysis of the code looks totally correct
- there's no bug there, at least related to hlist traversal and
kmem_cache_free(), since we are under rcu_read_lock() and thus hold off
the grace for freeing.

-- 
Jens Axboe

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