Re: BUG in: Driver core: convert block from raw kobjects to core devices (fwd)

[James Bottomley <James.Bottomley@SteelEye.com>]

On Wed, 2007-10-31 at 17:24 +0100, Kay Sievers wrote:
> On Wed, 2007-10-31 at 11:13 -0500, James Bottomley wrote:
> > On Wed, 2007-10-31 at 12:04 -0400, Alan Stern wrote:
> > > On Wed, 31 Oct 2007, James Bottomley wrote:
> > > 
> > > > > Yes, the queue is a child of the disk.
> > > > 
> > > > Right, so this goes gendisk->queue (-> meaning parent of, or takes
> > > > reference to)
> > > 
> > > No, no!  The _child_ takes an implicit reference to the _parent_, not 
> > > the other way around.
> > > 
> > > > > > The scsi_device has a ref to the queue
> > > > > 
> > > > > Yeah, while the queue is a grandchild of the scsi_device with the
> > > > > unified sysfs layout.
> > > > 
> > > > No, the scsi_device is a direct parent of the queue, so we have
> > > > 
> > > > scsi_device->queue
> > > 
> > > Wrong -- the gendisk is the direct parent of the queue.  The relevant 
> > > line is in ll_rw_blk.c:blk_register_queue():
> > > 
> > > 	q->kobj.parent = kobject_get(&disk->dev.kobj);
> > > 
> > > > > Yes, sounds right. We need to break that deleted-but-wait-for-cleanup at
> > > > > least at one of the devices involved.
> > > > 
> > > > But it's broken when the driver is unbound.  Diagrammatically it's:
> > > > 
> > > > scsi_disk -> scsi_device -> queue
> > > >           -> gendisk     ->
> > > > 
> > > > It's not circular, it's released when scsi_disk is released.  It can
> > > > become circular if there's some hidden dependency between any of the
> > > > components ... but I don't think there is.
> > > 
> > > Forget about the scsi_disk.  It isn't part of the problem.  Just 
> > > concentrate on the scsi_device, the gendisk, and the queue.  We have:
> > > 
> > > 	scsi_device <- gendisk <- queue <- scsi_device,
> > 
> > OK, so where does the gendisk get a reference to the scsi device?
> 
> In the unified sysfs layout where the silly and conceptual broken idea
> of "class devices" gets removed.
> Everything that has a "device" link today will just live below the
> device the "device" link points to. The whole current kernel is already
> converted to do this, besides the "raw kobject" gendisk's, and the SCSI
> subsystem. The gendisk patch is queued in Greg's tree (see subject of
> this mail), and the conversion from "struct class_device" to "struct
> device" for the whole SCSI directory is coming soon.
> 
> With the gendisk pointing to "driverfs_dev" ("device" link) it will
> become a child of the scsi_device.

OK, light beginning to go on now.

The problem is that you've fallen into the conceptual trap we tried very
hard to avoid in the initial go around of joining SCSI upper layer
drivers to gendisks.  That's why no gendisk references are held by the
mid-layer, only by the entities that represent the objects created by
upper layer drivers.

Doesn't this circularity now exist for everything?  Every device that
creates a queue has a reference to the queue, every queue has a
reference to its attached gendisk and now every gendisk has a reference
to the device creating the queue?  This doesn't look to be a SCSI
specific problem.

James