From: Yu Kuai <yukuai3@huawei.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 replacing et->tags will new allocated tags as well.

Noted there are still some long term problems that will require some
refactor to be fixed thoroughly[1].

[1] https://lore.kernel.org/all/20250815080216.410665-1-yukuai1@huaweicloud.com/
Fixes: f5a6604f7a44 ("block: fix lockdep warning caused by lock dependency in elv_iosched_store")

Signed-off-by: Yu Kuai <yukuai3@huawei.com>
---
 block/blk-mq-tag.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/block/blk-mq-tag.c b/block/blk-mq-tag.c
index d880c50629d6..5cffa5668d0c 100644
--- a/block/blk-mq-tag.c
+++ b/block/blk-mq-tag.c
@@ -622,6 +622,7 @@ int blk_mq_tag_update_depth(struct blk_mq_hw_ctx *hctx,
 			return -ENOMEM;
 
 		blk_mq_free_map_and_rqs(set, *tagsptr, hctx->queue_num);
+		hctx->queue->elevator->et->tags[hctx->queue_num] = new;
 		*tagsptr = new;
 	} else {
 		/*
-- 
2.39.2

On 8/19/25 6:59 AM, Yu Kuai wrote:
> From: Yu Kuai <yukuai3@huawei.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
nit: 's/exist/exit'

> free the tags again, causing kernel panic.
> 
> Fix this problem by replacing et->tags will new allocated tags as well.
nit: 's/will/with'

> 
> Noted there are still some long term problems that will require some
> refactor to be fixed thoroughly[1].
> 
> [1] https://lore.kernel.org/all/20250815080216.410665-1-yukuai1@huaweicloud.com/
> Fixes: f5a6604f7a44 ("block: fix lockdep warning caused by lock dependency in elv_iosched_store")
> 
> Signed-off-by: Yu Kuai <yukuai3@huawei.com>
> ---
>  block/blk-mq-tag.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/block/blk-mq-tag.c b/block/blk-mq-tag.c
> index d880c50629d6..5cffa5668d0c 100644
> --- a/block/blk-mq-tag.c
> +++ b/block/blk-mq-tag.c
> @@ -622,6 +622,7 @@ int blk_mq_tag_update_depth(struct blk_mq_hw_ctx *hctx,
>  			return -ENOMEM;
>  
>  		blk_mq_free_map_and_rqs(set, *tagsptr, hctx->queue_num);
> +		hctx->queue->elevator->et->tags[hctx->queue_num] = new;
>  		*tagsptr = new;
>  	} else {
>  		/*
Except the above minor nitpicking this change looks good to me:

Reviewed-by: Nilay Shroff<nilay@linux.ibm.com>

On Tue, Aug 19, 2025 at 09:29:17AM +0800, Yu Kuai wrote:
> From: Yu Kuai <yukuai3@huawei.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 replacing et->tags will new allocated tags as well.
> 
> Noted there are still some long term problems that will require some
> refactor to be fixed thoroughly[1].
> 
> [1] https://lore.kernel.org/all/20250815080216.410665-1-yukuai1@huaweicloud.com/
> Fixes: f5a6604f7a44 ("block: fix lockdep warning caused by lock dependency in elv_iosched_store")
> 
> Signed-off-by: Yu Kuai <yukuai3@huawei.com>
> ---
>  block/blk-mq-tag.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/block/blk-mq-tag.c b/block/blk-mq-tag.c
> index d880c50629d6..5cffa5668d0c 100644
> --- a/block/blk-mq-tag.c
> +++ b/block/blk-mq-tag.c
> @@ -622,6 +622,7 @@ int blk_mq_tag_update_depth(struct blk_mq_hw_ctx *hctx,
>  			return -ENOMEM;
>  
>  		blk_mq_free_map_and_rqs(set, *tagsptr, hctx->queue_num);
> +		hctx->queue->elevator->et->tags[hctx->queue_num] = new;
>  		*tagsptr = new;

Reviewed-by: Ming Lei <ming.lei@redhat.com>


Thanks,
Ming