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

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Message-ID: <4E2D678E.2010209@redhat.com>
Date:	Mon, 25 Jul 2011 14:54:38 +0200
From:	Zdenek Kabelac <zkabelac@...hat.com>
To:	Kay Sievers <kay.sievers@...y.org>
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

Dne 25.7.2011 14:17, Kay Sievers napsal(a):
> 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.

Well then I've probably not fully understand your idea here - I guess it would
then simpler written by you?


>> 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.

No - nothing so complex - kobject would have the list - and you would be able
to add some env param to this list - the nearest  kobject_uevent() would just
splice those parameters to the env list it wants to send (something like 10
lines of code). The only user would be probably dm so far - and it would check
it wants to send REMOVE - and in this case it would add env to kobject and
would skip kobject_uvent.

On the other hand, it would probably extended kobject struct size without big
use case - so Milan's solution that checks whether REMOVE has been already
sent and skip all future REMOVE events seems by far the simplest here.

I think your proposal also requires struct extension to store callback somewhere ?

>> 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.

del() looks like the best natural place here - and safe few lines of code ;)

Zdenek
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