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Date: Wed, 2 Apr 2008 14:01:13 +0300
From: "Pekka Enberg" <penberg@cs.helsinki.fi>
To: paulmck@linux.vnet.ibm.com
Subject: Re: kmemcheck caught read from freed memory (cfq_free_io_context)
Cc: "Ingo Molnar" <mingo@elte.hu>, "Jens Axboe" <jens.axboe@oracle.com>,
       "Peter Zijlstra" <a.p.zijlstra@chello.nl>,
       "Vegard Nossum" <vegard.nossum@gmail.com>,
       "Linux Kernel Mailing List" <linux-kernel@vger.kernel.org>
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Hi Paul,

On Wed, Apr 2, 2008 at 1:55 PM, Paul E. McKenney
<paulmck@linux.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