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

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Date:	Wed, 17 Aug 2016 17:14:22 +0800
From:	Fam Zheng <famz@...hat.com>
To:	Cornelia Huck <cornelia.huck@...ibm.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, 08/17 11:06, Cornelia Huck wrote:
> 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...)

Right, which is pre-exising.

> 
> 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.
> 

Of course, the plan is to write patches on top. I'm not cleaning up anything
here because I'm concerned callers may double free (and I didn't look hard into
that).

Fam