On Mon, May 12, 2025 at 02:43:38PM +0530, Nilay Shroff wrote:
> In __blk_mq_update_nr_hw_queues(), the current sequence involves:
>
> 1. unregistering sysfs/debugfs attributes
> 2. freeze the queue
> 3. reallocating the tag set
> 4. updating the queue map
> 5. reallocating hardware contexts
> 6. updating the elevator (which unfreeze the queue again)
> 7. re-register sysfs/debugfs attributes
>
> If tag set reallocation fails at step 3, the function skips steps 4–6
> and proceeds directly to step 7, re-registering the sysfs/debugfs
> attributes without unfreezing the queue first. This is incorrect and
> can lead to a system hang or lockdep splat, as the queue remains frozen
> and is never properly unfrozen.
>
> This patch addresses the issue by explicitly unfreezing the queue before
> re-registering the sysfs/debugfs attributes in the event of a tag set
> reallocation failure.
>
> Fixes: 9dc7a882ce96 ("block: move hctx debugfs/sysfs registering out of freezing queue")
> Signed-off-by: Nilay Shroff <nilay@xxxxxxxxxxxxx>
> ---
> block/blk-mq.c | 5 ++++-
> 1 file changed, 4 insertions(+), 1 deletion(-)
>
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index 4f79a9808fd1..cbc9a9f97a31 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -5002,8 +5002,11 @@ static void __blk_mq_update_nr_hw_queues(struct blk_mq_tag_set *set,
> list_for_each_entry(q, &set->tag_list, tag_set_list)
> blk_mq_freeze_queue_nomemsave(q);
>
> - if (blk_mq_realloc_tag_set_tags(set, nr_hw_queues) < 0)
> + if (blk_mq_realloc_tag_set_tags(set, nr_hw_queues) < 0) {
> + list_for_each_entry(q, &set->tag_list, tag_set_list)
> + blk_mq_unfreeze_queue_nomemrestore(q);
> goto reregister;
Reviewed-by: Ming Lei <ming.lei@xxxxxxxxxx>
Thanks,
Ming
