On Monday November 24, tj@xxxxxxxxxx wrote: > (cc'ing Greg) > > NeilBrown wrote: > > Currently md devices, once created, never disappear until the module > > is unloaded. This is essentially because the gendisk holds a > > reference to the mddev, and the mddev holds a reference to the > > gendisk, this a circular reference. > > > > If we drop the reference from mddev to gendisk, then we need to ensure > > that the mddev is destroyed when the gendisk is destroyed. However it > > is not possible to hook into the gendisk destruction process to enable > > this. > > > > So we drop the reference from the gendisk to the mddev and destroy the > > gendisk when the mddev gets destroyed. However this has a > > complication. > > Between the call > > __blkdev_get->get_gendisk->kobj_lookup->md_probe > > and the call > > __blkdev_get->md_open > > > > there is no obvious way to hold a reference on the mddev any more, so > > unless something is done, it will disappear and gendisk will be > > destroyed prematurely. > > > > Also, once we decide to destroy the mddev, there will be an unlockable > > moment before the gendisk is unlinked (blk_unregister_region) during > > which a new reference to the gendisk can be created. We need to > > ensure that this reference can not be used. i.e. the ->open must > > fail. > > Ah... I'm not really sure I'm following all of this correctly but would > it be possible to just add ->release to genhd and do regular reference > counting rather than this complex dancing? ->release was recently added > to cdev so it'll be nicely parallel. Maybe... If genhd.c:disk_release called e.g. disk->fops->final_put(disk) then I could possibly link in to that to destroy the md state when the gendisk finally disappears. When I want to kill the gendisk I would call blk_unregister_region directly (not through del_gendisk) to allow it to disappear. If md_probe then gets called before the final_put, I'd need to call blk_register_region again to re-install it. I think that would work. Would 'block_device_operations' be the right place for this 'final_put' or 'final_release' ?? Thanks. NeilBrown -- To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
