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

[Neil Brown <neilb@suse.de>]

On Monday November 24, tj@kernel.org 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