linux-kernel - Re: [PATCH 08/10] blk-mq: fix blk_mq_tags double free while nr

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Message-ID: <c5e63966-e7f6-4d82-9d66-3a0abccc9d17@linux.ibm.com>
Date: Sat, 16 Aug 2025 01:00:21 +0530
From: Nilay Shroff <nilay@...ux.ibm.com>
To: Yu Kuai <yukuai1@...weicloud.com>, axboe@...nel.dk, hare@...e.de,
        ming.lei@...hat.com
Cc: linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        yukuai3@...wei.com, yi.zhang@...wei.com, yangerkun@...wei.com,
        johnny.chenyi@...wei.com
Subject: Re: [PATCH 08/10] blk-mq: fix blk_mq_tags double free while
 nr_requests grown



On 8/15/25 1:32 PM, Yu Kuai wrote:
> From: Yu Kuai <yukuai3@...wei.com>
> 
> In the case user trigger tags grow by queue sysfs attribute nr_requests,
> hctx->sched_tags will be freed directly and replaced with a new
> allocated tags, see blk_mq_tag_update_depth().
> 
> The problem is that hctx->sched_tags is from elevator->et->tags, while
> et->tags is still the freed tags, hence later elevator exist will try to
> free the tags again, causing kernel panic.
> 
> Fix this problem by using new allocated elevator_tags, also convert
> blk_mq_update_nr_requests to void since this helper will never fail now.
> 
> Meanwhile, there is a longterm problem can be fixed as well:
> 
> If blk_mq_tag_update_depth() succeed for previous hctx, then bitmap depth
> is updated, however, if following hctx failed, q->nr_requests is not
> updated and the previous hctx->sched_tags endup bigger than q->nr_requests.
> 
> Fixes: f5a6604f7a44 ("block: fix lockdep warning caused by lock dependency in elv_iosched_store")
> Fixes: e3a2b3f931f5 ("blk-mq: allow changing of queue depth through sysfs")
> Signed-off-by: Yu Kuai <yukuai3@...wei.com>
> ---
>  block/blk-mq.c    | 19 ++++++-------------
>  block/blk-mq.h    |  4 +++-
>  block/blk-sysfs.c | 21 ++++++++++++++-------
>  3 files changed, 23 insertions(+), 21 deletions(-)
> 
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index 11c8baebb9a0..e9f037a25fe3 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -4917,12 +4917,12 @@ void blk_mq_free_tag_set(struct blk_mq_tag_set *set)
>  }
>  EXPORT_SYMBOL(blk_mq_free_tag_set);
>  
> -int blk_mq_update_nr_requests(struct request_queue *q, unsigned int nr)
> +void blk_mq_update_nr_requests(struct request_queue *q,
> +			       struct elevator_tags *et, unsigned int nr)
>  {
>  	struct blk_mq_tag_set *set = q->tag_set;
>  	struct blk_mq_hw_ctx *hctx;
>  	unsigned long i;
> -	int ret = 0;
>  
>  	blk_mq_quiesce_queue(q);
>  
> @@ -4946,24 +4946,17 @@ int blk_mq_update_nr_requests(struct request_queue *q, unsigned int nr)
>  				nr - hctx->sched_tags->nr_reserved_tags);
>  		}
>  	} else {
> -		queue_for_each_hw_ctx(q, hctx, i) {
> -			if (!hctx->tags)
> -				continue;
> -			ret = blk_mq_tag_update_depth(hctx, &hctx->sched_tags,
> -						      nr);
> -			if (ret)
> -				goto out;
> -		}
> +		blk_mq_free_sched_tags(q->elevator->et, set);

I think you also need to ensure that elevator tags are freed after we unfreeze
queue and release ->elevator_lock otherwise we may get into the lockdep splat
for pcpu_lock dependency on ->freeze_lock and/or ->elevator_lock. Please note 
that blk_mq_free_sched_tags internally invokes sbitmap_free which invokes 
free_percpu which acquires pcpu_lock. 
 
Thanks,
--Nilay