On Wed, 31 Oct 2007, James Bottomley wrote:
> On Wed, 2007-10-31 at 07:32 -0700, Greg KH wrote:
> > Hm, I seem to have missed the part in this thread where someone said
> > that it was valid to have a parent reference a child device. That's
> > just wrong and needs to be fixed. Is that in the scsi layer somewhere?
> > The block layer? It sure isn't in the driver core...
>
> This is the piece I'm still not clear on. It's something to do with the
> gendisk. I'd have to look in block, but I believe the queue takes a ref
> to the gendisk.
>
> The scsi_device has a ref to the queue and the scsi_disk (in sd) has a
> ref to both the scsi_device and the gendisk. That means, until sd is
> unbound and the scsi_disk released, there's an implied unbreakable
> reference chain.
>
> at least, I think that's what the problem is.
No, you haven't got it right.
Parent Child Grandchild
------ ----- ----------
scsi_device gendisk request_queue
The odd part is that the scsi_device holds a reference to the queue.
That creates a reference loop:
scsi_device holds ref to request_queue (done explicitly)
request_queue holds ref to gendisk (implicit, parent-child)
gendisk holds ref to scsi_device (implicit, parent-child)
The scsi_disk adds confusion to the picture but it doesn't make things
any worse than they already are.
Alan Stern
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