Re: [dm-devel] [PATCH 1/3] dm: don't grab target io reference in dm_zone_map_bio

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

On Mon, Apr 11, 2022 at 09:18:56AM +0900, Damien Le Moal wrote:
> On 4/9/22 02:12, Ming Lei wrote:
> > dm_zone_map_bio() is only called from __map_bio in which the io's
> > reference is grabbed already, and the reference won't be released
> > until the bio is submitted, so no necessary to do it dm_zone_map_bio
> > any more.
> 
> I do not think that this patch is correct. Removing the extra reference on
> the io can lead to problems if the io is completed in the target
> ->map(ti, clone) call or before dm_zone_map_bio_end() is called for the
> DM_MAPIO_SUBMITTED or DM_MAPIO_REMAPPED cases. dm_zone_map_bio_end() needs

__map_bio():
	...
	dm_io_inc_pending(io);
	...
	dm_zone_map_bio(tio);
	...

dm_submit_bio():
	dm_split_and_process_bio
		init_clone_info(&ci, md, map, bio)
			atomic_set(&io->io_count, 1)
		...
		__map_bio()
		...
		dm_io_dec_pending()

The target io can only be completed after the above line returns,
so the extra reference in dm_zone_map_bio() doesn't make any difference,
does it?


Thanks, 
Ming
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