I found out why it works on my system -- which still leaves open the
question of why it fails on yours.
Although the gendisk device is a child of the scsi_device, nevertheless
disk->kobj is not a child of the scsi_device's embedded kobject.
Instead it is a child of the static (!) block_depr kobject, which is
defined in block/genhd.c. Hence the disk's single reference to the
scsi_device is indeed dropped when the disk is unregistered, which
breaks the loop of circular references.
It works this way because I have CONFIG_SYSFS_DEPRECATED set; perhaps
you don't. If you compare the two versions of get_device_parent() in
drivers/base/core.c you'll see the difference (it's the dev_type ==
&disk_type case).
Alan Stern
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