On Monday November 24, viro@xxxxxxxxxxxxxxxxxx wrote: > On Mon, Nov 24, 2008 at 02:55:30PM +1100, NeilBrown wrote: > > 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. > > > > So: > > 1/ in md_probe we set a flag in the mddev (hold_active) which > > indicates that the array should be treated as active, even > > though there are no references, and no appearance of activity. > > This is cleared by md_release when the device is closed. > > This ensure that the gendisk will survive between md_probe and > > md_open. Thanks for the reply. > > That won't work. Note that you are not guaranteed that md_release() will be > called after md_probe(); there are failure exits in __blkdev_get() that do > not reach ->open() at all. I thought about those failure exits and concluded that they are the sort the almost never happen in practice (I think -ENOMEM is the only credible error) and the consequence is only that the gendisk will hang around a until some future open/close, so it is no worse that the current situation. Resolving that would be nice but I didn't feel up to any major surgery. > > What lifetime rules do you really want? I never liked the tricks pulled > by md wrt gendisk lifetimes and that might be a good time to sort that > out for good... I'm not sure what 'tricks' you are referring to. Can you elaborate? I want the gendisk to appear as soon as it is needed (not because I think that is necessarily a good idea, but it is legacy functionality that I don't think we can easily discard). And I want them to disappear when they contain no information and have nothing referring to them. > > What should happen to things like pending IO, etc. on array destruction? > AFAICS, that's the real question... Pending IO should not be a possibility thanks to the sync_blockdev call in __blkdev_put. During that last close, nothing can generate new IO, and any old IO will be flushed (won't it?). 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
