Re: [PATCH 1/2] block: fix lock ordering between the queue ->sysfs_lock and freeze-lock

[Christoph Hellwig <hch@lst.de>]

On Thu, Feb 06, 2025 at 06:52:36PM +0530, Nilay Shroff wrote:
> Yeah I tested with a multi namespace NVMe disk and lockdep didn't 
> complain. Agreed we need to hold up q->sysfs_lock for multiple 
> request queues at the same time and that may not be elegant, but 
> looking at the mess in __blk_mq_update_nr_hw_queues we may not
> have other choice which could help correct the lock order.

Odd, as it's usually very unhappy about nesting locks of the
same kind unless specifically annotated.

> Yes this is probably a good idea, that instead of using q->sysfs_lock 
> we may depend on q->tag_set->tag_list_lock here for sched/elevator updates
> as a fact that __blk_mq_update_nr_hw_queues already runs with tag_list_lock
> held.

Yes.

> But then it also requires using the same tag_list_lock instead of 
> current sysfs_lock while we update the scheduler from sysfs. But that's
> a trivial change.

Yes.  I think it's a good idea, but maybe wait a bit to see if Jens
or Ming also have opinions on this before starting the work.