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



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