Re: kmemcheck caught read from freed memory (cfq_free_io_context)

[Fabio Checconi]

> From: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
> 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@xxxxxxxxxx> 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@xxxxxxxxxxxxxxxx>
---
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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