linux-kernel - Re: [GIT PULL] kdbus for 4.1-rc1

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Date:	Wed, 29 Apr 2015 13:43:14 -0700 (PDT)
From:	David Lang <david@...g.hm>
To:	Andy Lutomirski <luto@...capital.net>
cc:	Austin S Hemmelgarn <ahferroin7@...il.com>,
	Harald Hoyer <harald@...hat.com>,
	Arnd Bergmann <arnd@...db.de>, Havoc Pennington <hp@...ox.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Jiri Kosina <jkosina@...e.cz>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Daniel Mack <daniel@...que.org>,
	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Lukasz Skalski <l.skalski@...sung.com>,
	"Theodore Ts'o" <tytso@....edu>, Tom Gundersen <teg@...m.no>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	David Herrmann <dh.herrmann@...il.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	John Stoffel <john@...ffel.org>,
	Djalal Harouni <tixxdz@...ndz.org>
Subject: Re: [GIT PULL] kdbus for 4.1-rc1

On Wed, 29 Apr 2015, Andy Lutomirski wrote:

> On Wed, Apr 29, 2015 at 1:15 PM, David Lang <david@...g.hm> wrote:
>> On Wed, 29 Apr 2015, Andy Lutomirski wrote:
>>
>>> On Wed, Apr 29, 2015 at 12:30 PM, Austin S Hemmelgarn
>>> <ahferroin7@...il.com> wrote:
>>>>
>>>> On 2015-04-29 14:54, Andy Lutomirski wrote:
>>>>>
>>>>>
>>>>> On Apr 29, 2015 5:48 AM, "Harald Hoyer" <harald@...hat.com> wrote:
>>>>>>
>>>>>>
>>>>>>
>>>>>>   * Being in the kernel closes a lot of races which can't be fixed with
>>>>>>     the current userspace solutions.  For example, with kdbus, there is
>>>>>> a
>>>>>>     way a client can disconnect from a bus, but do so only if no
>>>>>> further
>>>>>>     messages present in its queue, which is crucial for implementing
>>>>>>     race-free "exit-on-idle" services
>>>>>
>>>>>
>>>>>
>>>>> This can be implemented in userspace.
>>>>>
>>>>> Client to dbus daemon: may I exit now?
>>>>> Dbus daemon to client: yes (and no more messages) or no
>>>>>
>>>> Depending on how this is implemented, there would be a potential issue if
>>>> a
>>>> message arrived for the client after the daemon told it it could exit,
>>>> but
>>>> before it finished shutdown, in which case the message might get lost.
>>>>
>>>
>>> Then implement it the right way?  The client sends some kind of
>>> sequence number with its request.
>>
>>
>> so any app in the system can prevent any other app from exiting/restarting
>> by just sending it the equivalent of a ping over dbus?
>>
>> preventing an app from exiting because there are unhandled messages doesn't
>> mean that those messages are going to be handled, just that they will get
>> read and dropped on the floor by an app trying to exit. Sometimes you will
>> just end up with a hung app that can't process messages and needs to be
>> restarted, but can't be restarted because there are pending messages.
>
> I think this consideration is more or less the same whether it's
> handled in the kernel or in userspace, though.

If the justification for why this needs to be in the kernel is that you can't 
reliably prevent apps from exiting if there are pending messages, then the 
answer of "preventing apps from exiting if there are pending messages isn't a 
sane thing to try and do" is a direct counter to that justification for 
including it in the kernel.

David Lang
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