Re: [for-4.16 PATCH 1/2] block: cope with gendisk's 'queue' being added later

[Mike Snitzer <snitzer@redhat.com>]

On Tue, Jan 09 2018 at  6:41pm -0500,
Mike Snitzer <snitzer@redhat.com> wrote:

> On Tue, Jan 09 2018 at  6:04pm -0500,
> Bart Van Assche <Bart.VanAssche@wdc.com> wrote:
> 
> > On Tue, 2018-01-09 at 17:10 -0500, Mike Snitzer wrote:
> > > diff --git a/block/blk-sysfs.c b/block/blk-sysfs.c
> > > index 870484eaed1f..0b0dda8e2420 100644
> > > --- a/block/blk-sysfs.c
> > > +++ b/block/blk-sysfs.c
> > > @@ -919,8 +919,20 @@ int blk_register_queue(struct gendisk *disk)
> > >  	ret = 0;
> > >  unlock:
> > >  	mutex_unlock(&q->sysfs_lock);
> > > +
> > > +	/*
> > > +	 * Take an extra ref on queue which will be put on disk_release()
> > > +	 * so that it sticks around as long as @disk is there.
> > > +	 */
> > > +	WARN_ON_ONCE(!blk_get_queue(q));
> > > +
> > > +	WARN_ON(sysfs_create_link(&dev->kobj,
> > > +				  &q->backing_dev_info->dev->kobj,
> > > +				  "bdi"));
> > > +
> > >  	return ret;
> > >  }
> > > +EXPORT_SYMBOL_GPL(blk_register_queue);
> > 
> > Hello Mike,
> > 
> > So the sysfs_create_link() call is moved from register_disk() into
> > blk_register_queue() but the sysfs_remove_link() call stays in del_gendisk()?
> > Are you sure that you want this asymmetry?
> 
> My focus was on the add_disk() side of things, due to disk->queue
> possibly being NULL on add.  But on remove all was basically left
> unmodified (aside from removing the WARN_ON).
> 
> I dont think the asymmetry is a big deal but I can fix it.  I'll wait
> for more feedback before sending out a v2 though.

But while reviewing this asymetry I found that the sysfs_create_link()
that I moved to blk_register_queue() needs to be guarded against
GENHD_FL_HIDDEN -- I didn't notice the GENHD_FL_HIDDEN early return in
register_disk().  I'll get that fixed up.

But unrelated to my patch: I think I found another curious imbalance, in
current upstream code, relative to GENHD_FL_HIDDEN.
bdi_register_owner() is only called if !GENHD_FL_HIDDEN but
bdi_unregister() is called unconditionally.  Not sure what is needed to
address that issue because I'd have thought that the bdi would be needed
regardless of GENHD_FL_HIDDEN.  Christoph?