Re: [PATCH 3/3] lib/percpu-refcount: drain ->release() in perpcu_ref_exit()

[Ming Lei]

On Wed, Dec 14, 2022 at 04:16:51PM +0800, Hillf Danton wrote:
> On 14 Dec 2022 10:51:01 +0800 Ming Lei <ming.lei@xxxxxxxxxx>
> > The pattern of wait_event(percpu_ref_is_zero()) has been used in several
> 
> For example?

blk_mq_freeze_queue_wait() and target_wait_for_sess_cmds().

> 
> > kernel components, and this way actually has the following risk:
> > 
> > - percpu_ref_is_zero() can be returned just between
> >   atomic_long_sub_and_test() and ref->data->release(ref)
> > 
> > - given the refcount is found as zero, percpu_ref_exit() could
> >   be called, and the host data structure is freed
> > 
> > - then use-after-free is triggered in ->release() when the user host
> >   data structure is freed after percpu_ref_exit() returns
> 
> The race between exit and the release callback should be considered at the
> corresponding callsite, given the comment below, and closed for instance
> by synchronizing rcu.
> 
> /**
>  * percpu_ref_put_many - decrement a percpu refcount
>  * @ref: percpu_ref to put
>  * @nr: number of references to put
>  *
>  * Decrement the refcount, and if 0, call the release function (which was passed
>  * to percpu_ref_init())
>  *
>  * This function is safe to call as long as @ref is between init and exit.
>  */

Not sure if the above comment implies that the callsite should cover the
race.

But blk-mq can really avoid the trouble by using the existed call_rcu():


diff --git a/block/blk-core.c b/block/blk-core.c
index 3866b6c4cd88..9321767470dc 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -254,14 +254,15 @@ EXPORT_SYMBOL_GPL(blk_clear_pm_only);
 
 static void blk_free_queue_rcu(struct rcu_head *rcu_head)
 {
-	kmem_cache_free(blk_requestq_cachep,
-			container_of(rcu_head, struct request_queue, rcu_head));
+	struct request_queue *q = container_of(rcu_head,
+			struct request_queue, rcu_head);
+
+	percpu_ref_exit(&q->q_usage_counter);
+	kmem_cache_free(blk_requestq_cachep, q);
 }
 
 static void blk_free_queue(struct request_queue *q)
 {
-	percpu_ref_exit(&q->q_usage_counter);
-
 	if (q->poll_stat)
 		blk_stat_remove_callback(q, q->poll_cb);
 	blk_stat_free_callback(q->poll_cb);


Thanks, 
Ming