From mboxrd@z Thu Jan 1 00:00:00 1970 From: Tejun Heo Subject: Re: [PATCH 1/2] md: make devices disappear when they are no longer needed. Date: Mon, 24 Nov 2008 13:18:51 +0900 Message-ID: <492A2B2B.7030606@kernel.org> References: <20081124035516.3465.66413.stgit@notabene.brown> <20081124035530.3465.26724.stgit@notabene.brown> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <20081124035530.3465.26724.stgit@notabene.brown> Sender: linux-raid-owner@vger.kernel.org To: NeilBrown Cc: linux-kernel@vger.kernel.org, linux-raid@vger.kernel.org, Al Viro , Doug Ledford , Greg KH List-Id: linux-raid.ids (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. Thanks. -- tejun