On 8/15/25 1:32 PM, Yu Kuai wrote:
> From: Yu Kuai <yukuai3@xxxxxxxxxx>
>
> 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@xxxxxxxxxx>
> ---
> 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
