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block: unfreeze queue if realloc tag set fails during nr_hw_queues update

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>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20250512092952.135887-1-nilay@linux.ibm.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

authored by

Nilay Shroff and committed by
Jens Axboe 2d8951ae 824afb9b

+4 -1
+4 -1
block/blk-mq.c
··· 5002 5002 list_for_each_entry(q, &set->tag_list, tag_set_list) 5003 5003 blk_mq_freeze_queue_nomemsave(q); 5004 5004 5005 - if (blk_mq_realloc_tag_set_tags(set, nr_hw_queues) < 0) 5005 + if (blk_mq_realloc_tag_set_tags(set, nr_hw_queues) < 0) { 5006 + list_for_each_entry(q, &set->tag_list, tag_set_list) 5007 + blk_mq_unfreeze_queue_nomemrestore(q); 5006 5008 goto reregister; 5009 + } 5007 5010 5008 5011 fallback: 5009 5012 blk_mq_update_queue_map(set);