Re: [PATCH 1/2] md: make devices disappear when they are no longerneeded.

[Al Viro]

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.

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.

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...

What should happen to things like pending IO, etc. on array destruction?
AFAICS, that's the real question...
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