(cc'ing Jens) Neil Brown wrote: > 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' ?? I suppose so. Maybe just void (*release)(struct gendisk *) but Jens is the maintainer. Jens, what do you think? Thanks. -- tejun -- 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
