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    SubjectRe: [PATCH 1/2] md: make devices disappear when they are no longer needed.
    On Monday November 24, 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.


    If genhd.c:disk_release called e.g.

    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' ??



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