linux-kernel - Re: [dm-devel] linux-next - WARNING: at fs/block_dev.c:824 bd_link_disk

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Date:	Thu, 13 Jan 2011 17:10:02 +0100
From:	Kay Sievers <kay.sievers@...y.org>
To:	Karel Zak <kzak@...hat.com>
Cc:	Tejun Heo <tj@...nel.org>, Milan Broz <mbroz@...hat.com>,
	device-mapper development <dm-devel@...hat.com>,
	"Jun'ichi Nomura" <j-nomura@...jp.nec.com>,
	Valdis.Kletnieks@...edu, linux-kernel@...r.kernel.org,
	linux-raid@...r.kernel.org,
	Alexander Viro <viro@...iv.linux.org.uk>,
	linux-fsdevel@...r.kernel.org
Subject: Re: [dm-devel] linux-next - WARNING: at fs/block_dev.c:824 bd_link_disk_holder+0x92/0x1ac()

On Thu, Jan 13, 2011 at 16:59, Karel Zak <kzak@...hat.com> wrote:
> On Thu, Jan 13, 2011 at 03:43:38PM +0100, Kay Sievers wrote:
>> On Thu, Jan 13, 2011 at 15:30, Tejun Heo <tj@...nel.org> wrote:
>> > On Thu, Jan 13, 2011 at 3:25 PM, Milan Broz <mbroz@...hat.com> wrote:
>> >> Maybe, but this was not invented in DM/MD camp:-)
>> >> Probably Kay or Greg can answer why it was done this way?
>>
>> It's not from Greg or Kay. It just appeared some day in the context of dm. :)
>>
>> And yes, symlinks *look* nice and simple for the outside, but they are
>> not, and have all sorts of problems like non-atomic updates, make it
>
>  Sounds like sysfs implementation problem, right?

It's a normal multi-file problem. It can by-definition not be atomic
without doing really weird locking things.

>  If there is noway to fix sysfs then we can add a generic ioctl or
>  /sys/block/<device>/{slave,holder}_list files with list of
>  holders/slaves.

Yeah, we've been there with the btrfs problem. For btrfs it woud
probably need to be something statfs()-like.

>  But please, don't force userspace to use *claimer-specific*
>  methods to answer *generic questions* like slave/holder dependencies
>  between devices.

The links exist only for dm and md so far, I think. It's the classical
multiple-parents-in-a-tree problem. We have that for bonded network
devices and some IO buses too. There is no nice representation for
these reversed-trees-in-the-tree so far.

Kay
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