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SubjectRe: kmemcheck caught read from freed memory (cfq_free_io_context)
> From: Peter Zijlstra <a.p.zijlstra@chello.nl>
> Date: Wed, Apr 02, 2008 12:59:21PM +0200
>
> On Wed, 2008-04-02 at 03:55 -0700, Paul E. McKenney wrote:
> > On Wed, Apr 02, 2008 at 09:28:46AM +0200, Ingo Molnar wrote:
> > >
> > > * Jens Axboe <jens.axboe@oracle.com> wrote:
> > >
> > > > On Wed, Apr 02 2008, Pekka J Enberg wrote:
> > > > > On Wed, 2 Apr 2008, Jens Axboe wrote:
> > > > > > Good catch, I wonder why it didn't complain in my testing. I've added a
> > > > > > patch to fix that, please see it here:
> > > > >
> > > > > You probably don't have kmemcheck in your kernel ;-)
> > > >
> > > > Ehm no, you are right :)
> > >
> > > ... and you can get kmemcheck by testing on x86.git/latest:
> > >
> > > http://people.redhat.com/mingo/x86.git/README
> > >
> > > ;-)
> >
> > 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().
>
> I don't think it does.
>
> 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 had the same problem while debugging a cfq-derived i/o scheduler,
and I found nothing preventing the reuse of the freed memory.
The patch below seemed to fix the logic.

Signed-off-by: Fabio Checconi <fabio@gandalf.sssup.it>
---
diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
index 0f962ec..f26da2b 100644
--- a/block/cfq-iosched.c
+++ b/block/cfq-iosched.c
@@ -1143,24 +1143,37 @@ static void cfq_put_queue(struct cfq_queue *cfqq)
}

/*
- * Call func for each cic attached to this ioc. Returns number of cic's seen.
+ * Call func for each cic attached to this ioc.
*/
-static unsigned int
+static void
call_for_each_cic(struct io_context *ioc,
void (*func)(struct io_context *, struct cfq_io_context *))
{
struct cfq_io_context *cic;
struct hlist_node *n;
- int called = 0;

rcu_read_lock();
- hlist_for_each_entry_rcu(cic, n, &ioc->cic_list, cic_list) {
+ hlist_for_each_entry_rcu(cic, n, &ioc->cic_list, cic_list)
func(ioc, cic);
- called++;
- }
rcu_read_unlock();
+}
+
+static void cfq_cic_free_rcu(struct rcu_head *head)
+{
+ struct cfq_io_context *cic;
+
+ cic = container_of(head, struct cfq_io_context, rcu_head);
+
+ kmem_cache_free(cfq_ioc_pool, cic);
+ elv_ioc_count_dec(ioc_count);
+
+ if (ioc_gone && !elv_ioc_count_read(ioc_count))
+ complete(ioc_gone);
+}

- return called;
+static void cfq_cic_free(struct cfq_io_context *cic)
+{
+ call_rcu(&cic->rcu_head, cfq_cic_free_rcu);
}

static void cic_free_func(struct io_context *ioc, struct cfq_io_context *cic)
@@ -1174,24 +1187,18 @@ static void cic_free_func(struct io_context *ioc, struct cfq_io_context *cic)
hlist_del_rcu(&cic->cic_list);
spin_unlock_irqrestore(&ioc->lock, flags);

- kmem_cache_free(cfq_ioc_pool, cic);
+ cfq_cic_free(cic);
}

static void cfq_free_io_context(struct io_context *ioc)
{
- int freed;
-
/*
- * ioc->refcount is zero here, so no more cic's are allowed to be
- * linked into this ioc. So it should be ok to iterate over the known
- * list, we will see all cic's since no new ones are added.
+ * ioc->refcount is zero here, or we are called from elv_unregister(),
+ * so no more cic's are allowed to be linked into this ioc. So it
+ * should be ok to iterate over the known list, we will see all cic's
+ * since no new ones are added.
*/
- freed = call_for_each_cic(ioc, cic_free_func);
-
- elv_ioc_count_mod(ioc_count, -freed);
-
- if (ioc_gone && !elv_ioc_count_read(ioc_count))
- complete(ioc_gone);
+ call_for_each_cic(ioc, cic_free_func);
}

static void cfq_exit_cfqq(struct cfq_data *cfqd, struct cfq_queue *cfqq)
@@ -1458,15 +1465,6 @@ cfq_get_queue(struct cfq_data *cfqd, int is_sync, struct io_context *ioc,
return cfqq;
}

-static void cfq_cic_free(struct cfq_io_context *cic)
-{
- kmem_cache_free(cfq_ioc_pool, cic);
- elv_ioc_count_dec(ioc_count);
-
- if (ioc_gone && !elv_ioc_count_read(ioc_count))
- complete(ioc_gone);
-}
-
/*
* We drop cfq io contexts lazily, so we may find a dead one.
*/
@@ -2138,7 +2136,7 @@ static int __init cfq_slab_setup(void)
if (!cfq_pool)
goto fail;

- cfq_ioc_pool = KMEM_CACHE(cfq_io_context, SLAB_DESTROY_BY_RCU);
+ cfq_ioc_pool = KMEM_CACHE(cfq_io_context, 0);
if (!cfq_ioc_pool)
goto fail;

@@ -2286,7 +2284,6 @@ static void __exit cfq_exit(void)
smp_wmb();
if (elv_ioc_count_read(ioc_count))
wait_for_completion(ioc_gone);
- synchronize_rcu();
cfq_slab_kill();
}

diff --git a/include/linux/iocontext.h b/include/linux/iocontext.h
index 1b4ccf2..50e448c 100644
--- a/include/linux/iocontext.h
+++ b/include/linux/iocontext.h
@@ -54,6 +54,8 @@ struct cfq_io_context {

void (*dtor)(struct io_context *); /* destructor */
void (*exit)(struct io_context *); /* called on task exit */
+
+ struct rcu_head rcu_head;
};

/*

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