linux-kernel - Re: [PATCH 4/4] block, bfq: update pos_root for idle bfq_queue in bfq_bfqq

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Message-ID: <20211221115001.GD24748@quack2.suse.cz>
Date:   Tue, 21 Dec 2021 12:50:01 +0100
From:   Jan Kara <jack@...e.cz>
To:     Yu Kuai <yukuai3@...wei.com>
Cc:     tj@...nel.org, axboe@...nel.dk, paolo.valente@...aro.org,
        jack@...e.cz, fchecconi@...il.com, avanzini.arianna@...il.com,
        cgroups@...r.kernel.org, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org, yi.zhang@...wei.com
Subject: Re: [PATCH 4/4] block, bfq: update pos_root for idle bfq_queue in
 bfq_bfqq_move()

On Tue 21-12-21 11:21:35, Yu Kuai wrote:
> 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.
> 
> Fixes: e21b7a0b9887 ("block, bfq: add full hierarchical scheduling and cgroups support")
> Signed-off-by: Yu Kuai <yukuai3@...wei.com>

I'm just wondering, how can it happen that !bfq_bfqq_busy() queue is in
pos_tree? Because bfq_remove_request() takes care to remove bfqq from the
pos_tree...

								Honza

> ---
>  block/bfq-cgroup.c | 14 +++++++++-----
>  1 file changed, 9 insertions(+), 5 deletions(-)
> 
> diff --git a/block/bfq-cgroup.c b/block/bfq-cgroup.c
> index 8e8cf6b3d946..822dd28ecf53 100644
> --- a/block/bfq-cgroup.c
> +++ b/block/bfq-cgroup.c
> @@ -677,7 +677,6 @@ 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(old_parent);
>  
>  	if (entity->parent &&
>  	    entity->parent->last_bfqq_created == bfqq)
> @@ -690,11 +689,16 @@ 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);
> -- 
> 2.31.1
> 
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR