Re: [dm-devel] patch to dm.c to delay add_disk call for new mapped device till a fter device's map is loaded

[Alasdair G Kergon <agk redhat com>]

On Thu, Nov 10, 2005 at 01:53:53PM -0500, goggin, edward wrote:
> Patch to dm.c to delay the add_disk call for a newly created mapped device
> until after the mapped device's map is loaded for the first time in dev_load
> so that an a hotplug/uevent handler will not fail to acquire information
> about
> the mapped device's map.
 
I'm not keen on this one yet - I can see the problem, but uevent is asynchronous
so I'm not sure where the right place to fix this is.
[By the time the handler runs, anything could have happened to the device's 
state - the device could even have been deleted.  So when the handler runs it
can make *no* assumptions about the state of the device.  Particularly as
uevents can get lost.]

If the handler wants to know what type of table, then it should be notified
every time a new table goes live - separately from when the device is created.

If the uuid holds the 'creating subsystem' as a prefix then that's sufficient to
identify it as multipath or EVMS or LVM2 etc. rather than looking for a map.

And for snapshots, issuing a uevent when a table is activated may risk problems
- it needs thinking through.

Alasdair