Re: [PATCH] block: unfreeze queue if realloc tag set fails during nr_hw_queues update

[Ming Lei <ming.lei@redhat.com>]

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@linux.ibm.com>
> ---
>  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@redhat.com>

Thanks,
Ming