Re: [PATCH V2] blk-mq: fix lockdep warning in __blk_mq_update_nr_hw_queues

[Nilay Shroff <nilay@linux.ibm.com>]

On 8/15/25 6:47 PM, Ming Lei wrote:
> Commit 5989bfe6ac6b ("block: restore two stage elevator switch while
> running nr_hw_queue update") reintroduced a lockdep warning by calling
> blk_mq_freeze_queue_nomemsave() before switching the I/O scheduler.
> 
> The function blk_mq_elv_switch_none() calls elevator_change_done().
> Running this while the queue is frozen causes a lockdep warning.
> 
> Fix this by reordering the operations: first, switch the I/O scheduler
> to 'none', and then freeze the queue. This ensures that elevator_change_done()
> is not called on an already frozen queue. And this way is safe because
> elevator_set_none() does freeze queue before switching to none.
> 
> Also we still have to rely on blk_mq_elv_switch_back() for switching
> back, and it has to cover unfrozen queue case.
> 
> Cc: Nilay Shroff <nilay@linux.ibm.com>
> Cc: Yu Kuai <yukuai3@huawei.com>
> Fixes: 5989bfe6ac6b ("block: restore two stage elevator switch while running nr_hw_queue update")
> Signed-off-by: Ming Lei <ming.lei@redhat.com>
> ---

Looks good to me:
Reviewed-by: Nilay Shroff <nilay@linux.ibm.com>