Re: [PATCH 1/3] Send KOBJ_ADD event after dm resume ioctl.

From: Alasdair G Kergon <agk@redhat.com>
To: David Zeuthen <zeuthen@gmail.com>
Cc: device-mapper development <dm-devel@redhat.com>
Subject: Re: [PATCH 1/3] Send KOBJ_ADD event after dm resume	ioctl.
Date: Fri, 19 Mar 2010 14:34:12 +0000	[thread overview]
Message-ID: <20100319143412.GE919@agk-dp.fab.redhat.com> (raw)
In-Reply-To: <7b13a0f81003190658i78664328ocb94aa0048054a17@mail.gmail.com>

On Fri, Mar 19, 2010 at 09:58:00AM -0400, David Zeuthen wrote:
> I think the problem is the that fact that 3rd party user space
> opens the device before it is ready (e.g. just after ADD but before
> the first CHANGE) makes things fall over.
> This short-coming is what needs to get fixed, I think - it's very
> fragile this way and since any random user / package can add
> rules to open the device on add events, said user / package can
> make device-mapper fail. Which doesn't exactly strike me
> as robust behavior.

And we suggested two potential solutions:

  1 - change the kernel so the ADD event doesn't arrive until the device is
ready for use.     [plus equivalent change for REMOVE]

      Advantage: the dm device handling looks more like a real disk so we have
less 'special case' code.  /dev then only indexes "dm devices ready to be used"
rather than "dm devices registered in the kernel"

      Disadvantage: breaks the currently-simple kobject/sysfs/dev linkage (as
per Kay's earlier mail)

  2 - several changes to the way udev rules are handled so we can choose to
ignore events and make no changes to /dev, so we can override rules other
packages insert without requiring dm-specific checks adding to them all, and
probably some of the other things we've discussed on these various threads.

Alasdair