Re: [PATCH RFC] block, bfq: update pos_root for idle bfq_queue in bfq_bfqq_move()

["yukuai (C)" <yukuai3@huawei.com>]

在 2021/12/10 16:16, Yu Kuai 写道:
> During code review, we found that if bfqq is not busy in
> bfq_bfqq_move(), bfq_pos_tree_add_move() won't be called for the bfqq,
> thus bfqq->pos_root still points to the old bfqg. However, the ref
> that bfqq hold for the old bfqg will be released, so it's possible
> that the old bfqg can be freed. This is problematic because the freed
> bfqg can still be accessed by bfqq->pos_root.
> 
> Fix the problem by calling bfq_pos_tree_add_move() for idle bfqq
> as well.
> 
Friendly ping ...
> Signed-off-by: Yu Kuai <yukuai3@huawei.com>
> ---
>   block/bfq-cgroup.c | 15 ++++++++++-----
>   1 file changed, 10 insertions(+), 5 deletions(-)
> 
> diff --git a/block/bfq-cgroup.c b/block/bfq-cgroup.c
> index 24a5c5329bcd..85f34c29b909 100644
> --- a/block/bfq-cgroup.c
> +++ b/block/bfq-cgroup.c
> @@ -645,6 +645,7 @@ void bfq_bfqq_move(struct bfq_data *bfqd, struct bfq_queue *bfqq,
>   		   struct bfq_group *bfqg)
>   {
>   	struct bfq_entity *entity = &bfqq->entity;
> +	struct bfq_group *old_parent = bfqq_group(bfqq);
>   
>   	/*
>   	 * Get extra reference to prevent bfqq from being freed in
> @@ -666,7 +667,6 @@ void bfq_bfqq_move(struct bfq_data *bfqd, struct bfq_queue *bfqq,
>   		bfq_deactivate_bfqq(bfqd, bfqq, false, false);
>   	else if (entity->on_st_or_in_serv)
>   		bfq_put_idle_entity(bfq_entity_service_tree(entity), entity);
> -	bfqg_and_blkg_put(bfqq_group(bfqq));
>   
>   	if (entity->parent &&
>   	    entity->parent->last_bfqq_created == bfqq)
> @@ -679,11 +679,16 @@ void bfq_bfqq_move(struct bfq_data *bfqd, struct bfq_queue *bfqq,
>   	/* pin down bfqg and its associated blkg  */
>   	bfqg_and_blkg_get(bfqg);
>   
> -	if (bfq_bfqq_busy(bfqq)) {
> -		if (unlikely(!bfqd->nonrot_with_queueing))
> -			bfq_pos_tree_add_move(bfqd, bfqq);
> +	/*
> +	 * Don't leave the pos_root to old bfqg, since the ref to old bfqg will
> +	 * be released and the bfqg might be freed.
> +	 */
> +	if (unlikely(!bfqd->nonrot_with_queueing))
> +		bfq_pos_tree_add_move(bfqd, bfqq);
> +	bfqg_and_blkg_put(old_parent);
> +
> +	if (bfq_bfqq_busy(bfqq))
>   		bfq_activate_bfqq(bfqd, bfqq);
> -	}
>   
>   	if (!bfqd->in_service_queue && !bfqd->rq_in_driver)
>   		bfq_schedule_dispatch(bfqd);
>