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

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Date:	Wed, 2 Apr 2008 09:08:09 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Pekka Enberg <penberg@...helsinki.fi>
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)

On Wed, Apr 02, 2008 at 02:01:13PM +0300, 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.

Would the following be an appropriate fix?  It seems to me to be in
the same spirit as the existing check for s->ctor.

Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
---

 slub_kmemcheck.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/slub_kmemcheck.c b/mm/slub_kmemcheck.c
index 8620a8b..e07f62a 100644
--- a/mm/slub_kmemcheck.c
+++ b/mm/slub_kmemcheck.c
@@ -93,6 +93,6 @@ kmemcheck_slab_alloc(struct kmem_cache *s, gfp_t gfpflags, void *object)
 void
 kmemcheck_slab_free(struct kmem_cache *s, void *object)
 {
-	if (!s->ctor)
+	if (!s->ctor && !(s->flags & SLAB_DESTROY_BY_RCU))
 		kmemcheck_mark_freed(object, s->objsize);
 }
--
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