linux-kernel - Re: [dm-devel] [PATCH] Reduce number of KOBJ

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Date:	Mon, 25 Jul 2011 14:17:01 +0200
From:	Kay Sievers <kay.sievers@...y.org>
To:	Zdenek Kabelac <zkabelac@...hat.com>
Cc:	device-mapper development <dm-devel@...hat.com>,
	Greg KH <greg@...ah.com>, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [dm-devel] [PATCH] Reduce number of KOBJ_REMOVE events

On Mon, Jul 25, 2011 at 12:12, Zdenek Kabelac <zkabelac@...hat.com> wrote:
> Dne 25.7.2011 02:18, Kay Sievers napsal(a):
>> On Fri, 2011-07-22 at 16:22 +0200, Zdenek Kabelac wrote:
>>
>>> For now udev recieves 3 event for removal of DM logical volumes. (1 for
>>> bdi and 2 for same block kobject). Reason is dm device generates its
>>> own kobject event with approriate env parameter and block layer sends
>>> another KOBJ_REMOVE event on its own unconditionaly for the same
>>> kobject. As for now only the kobject cleanup checks that the REMOVE
>>> event has been already sent and avoids duplicate REMOVE event.
>>
>>> The patch for kobject_uevent_env() which has been testing for duplicate
>>> REMOVE event did not passed into the mainline (yet?):
>>
>> No, it's wasn't merged. Subsystems should really not send their own
>> 'add' or 'remove' events. These are properties of the driver core.
>>
>>> I'm proposing alternative way around to always use kobject cleanup
>>> routine for sending REMOVE event if it was not send by the module - so
>>> it makes the code few lines shorter.
>>
>> The events the core creates are only sent out at release() not at del(),
>> so we would delay 'remove' events when we keep the device pinned but
>> it's not valid anymore. We can not do that today, we would need to move
>> the core-created 'remove' events to del().
>>
>> For device-mapper, I would prefer to add a '.dev_uevent' callback to the
>> 'block' class let this callback check 'struct block_device_operations'
>> for a possibly specified '.uevent' callback and call it.
>>
>> Then have 'dm_blk_dops' add '.uevent' and let the core call into the dm
>> code to the needed properties to the 'remove' event, instead of sending
>> its own, and see the duplication.
>
> Sounds like complex solution

I don't think so, It's clean, ~30 lines long, and technically correct, I expect.

> maybe it would be easier to just register some
> environment variable on dm code side - like kobject_add_env() - so it would
> take envs from this internal kobject list and after sending uevent it would
> implicitly clear this list.

So we would keep allocated per-event-type variables in the kobject, to
send when 'remove' is finally called? The callbacks are just much
simpler , I guess.

> So in dm case  dm-uevent would just register  env(cookie) for KOBJ_REMOVE and
> would left kobject_uevent() on block layer ?
>
> Also I'm aware that remove event would be delayed by leaving it on
> kobject_cleanup(), but since you mentioned 'del()' as a better place - why not
> move this implicit uvent call there.

It's probably not wrong to do that, but I don't remember now why we
added it to release() that time.

> since most kernel driver writers
> probably do not want to be bothered with uvents?

I guess, most drivers use devices not raw kobjects, which send events
on their own, and don't need anything here.

Kay
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