linux-kernel - Re: [PATCH 06/15] genhd: Add return code to device_add

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Message-Id: <20160817110645.45370f57.cornelia.huck@de.ibm.com>
Date:	Wed, 17 Aug 2016 11:06:45 +0200
From:	Cornelia Huck <cornelia.huck@...ibm.com>
To:	Fam Zheng <famz@...hat.com>
Cc:	linux-kernel@...r.kernel.org, Jens Axboe <axboe@...nel.dk>,
	linux-block@...r.kernel.org,
	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
	"Michael S. Tsirkin" <mst@...hat.com>,
	Michael Ellerman <mpe@...erman.id.au>,
	virtualization@...ts.linux-foundation.org,
	linux-nvme@...ts.infradead.org, "Ed L. Cashin" <ed.cashin@....org>,
	Keith Busch <keith.busch@...el.com>,
	Minchan Kim <minchan@...nel.org>,
	Paul Mackerras <paulus@...ba.org>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	linux-mtd@...ts.infradead.org,
	Brian Norris <computersforpeace@...il.com>,
	linuxppc-dev@...ts.ozlabs.org,
	David Woodhouse <dwmw2@...radead.org>,
	Nitin Gupta <ngupta@...are.org>
Subject: Re: [PATCH 06/15] genhd: Add return code to device_add_disk

On Wed, 17 Aug 2016 16:48:23 +0800
Fam Zheng <famz@...hat.com> wrote:

> On Wed, 08/17 10:49, Cornelia Huck wrote:
> > On Wed, 17 Aug 2016 15:15:06 +0800
> > Fam Zheng <famz@...hat.com> wrote:
> > 
> > > @@ -613,10 +614,8 @@ void device_add_disk(struct device *parent, struct gendisk *disk)
> > >  	disk->flags |= GENHD_FL_UP;
> > > 
> > >  	retval = blk_alloc_devt(&disk->part0, &devt);
> > > -	if (retval) {
> > > -		WARN_ON(1);
> > > -		return;
> > > -	}
> > > +	if (retval)
> > > +		goto fail;
> > >  	disk_to_dev(disk)->devt = devt;
> > > 
> > >  	/* ->major and ->first_minor aren't supposed to be
> > > @@ -625,16 +624,26 @@ void device_add_disk(struct device *parent, struct gendisk *disk)
> > >  	disk->major = MAJOR(devt);
> > >  	disk->first_minor = MINOR(devt);
> > > 
> > > -	disk_alloc_events(disk);
> > > +	retval = disk_alloc_events(disk);
> > > +	if (retval)
> > > +		goto fail;
> > > 
> > >  	/* Register BDI before referencing it from bdev */
> > >  	bdi = &disk->queue->backing_dev_info;
> > > -	bdi_register_owner(bdi, disk_to_dev(disk));
> > > +	retval = bdi_register_owner(bdi, disk_to_dev(disk));
> > > +	if (retval)
> > > +		goto fail;
> > > 
> > > -	blk_register_region(disk_devt(disk), disk->minors, NULL,
> > > -			    exact_match, exact_lock, disk);
> > > -	register_disk(parent, disk);
> > > -	blk_register_queue(disk);
> > > +	retval = blk_register_region(disk_devt(disk), disk->minors, NULL,
> > > +				     exact_match, exact_lock, disk);
> > > +	if (retval)
> > > +		goto fail;
> > > +	retval = register_disk(parent, disk);
> > > +	if (retval)
> > > +		goto fail;
> > > +	retval = blk_register_queue(disk);
> > > +	if (retval)
> > > +		goto fail;
> > > 
> > >  	/*
> > >  	 * Take an extra ref on queue which will be put on disk_release()
> > > @@ -644,10 +653,20 @@ void device_add_disk(struct device *parent, struct gendisk *disk)
> > > 
> > >  	retval = sysfs_create_link(&disk_to_dev(disk)->kobj, &bdi->dev->kobj,
> > >  				   "bdi");
> > > +	if (retval)
> > > +		goto fail;
> > > +
> > > +	retval = disk_add_events(disk);
> > > +	if (retval)
> > > +		goto fail;
> > > +
> > > +	retval = blk_integrity_add(disk);
> > > +	if (retval)
> > > +		goto fail;
> > > +	return 0;
> > > +fail:
> > >  	WARN_ON(retval);
> > > -
> > > -	disk_add_events(disk);
> > > -	blk_integrity_add(disk);
> > > +	return retval;
> > >  }
> > 
> > Noticed this when trying to figure out whether the error handling in
> > virtio_blk was correct:
> > 
> > Shouldn't you try to cleanup/rewind so that any structures are in a
> > sane state after failure? The caller doesn't know where device_add_disk
> > failed, and calling del_gendisk unconditionally like virtio_blk does is
> > probably not the right thing to do (at the very least, I don't think
> > unregistering a device that has not been registered is likely to work).
> > 
> 
> Yes, I think all callers need to be reviewed before device_add_disk do the
> clean up on error. For this patchset I wanted to keep the change small.

But do the callers even have a chance to do this correctly right now?
They will either clean up too much, or too little. ('Too little' is
probably the more common case, given that you just added error
propagation...)

Can you make del_gendisk handle devices partially setup via
device_add_disk in all cases? Then you could mandate pairing
device_add_disk with del_gendisk in all cases, error or not, and you
should have a better chance on avoiding introducing new errors.