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

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Message-ID: <ca37dc6598bb5728423fd902ac5322f9.squirrel@neil.brown.name>
Date:	Mon, 24 Nov 2008 17:10:51 +1100 (EST)
From:	"NeilBrown" <neilb@...e.de>
To:	"Tejun Heo" <tj@...nel.org>
Cc:	linux-kernel@...r.kernel.org, linux-raid@...r.kernel.org,
	"Al Viro" <viro@...iv.linux.org.uk>,
	"Doug Ledford" <dledford@...hat.com>, "Greg KH" <greg@...ah.com>,
	"Jens Axboe" <jens.axboe@...cle.com>
Subject: Re: [PATCH 1/2] md: make devices disappear when they are no longer 
     needed.

On Mon, November 24, 2008 4:34 pm, Tejun Heo wrote:
> (cc'ing Jens)
>
> Neil Brown wrote:
>> On Monday November 24, tj@...nel.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' ??
>
> I suppose so.  Maybe just void (*release)(struct gendisk *) but Jens is
> the maintainer.  Jens, what do you think?

Note that we already have 'release' in block_device_operations.  It is
called on last close rather than last put.

NeilBrown

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