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

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Date:	Wed, 29 Apr 2015 18:34:18 -0400
From:	"John Stoffel" <john@...ffel.org>
To:	Austin S Hemmelgarn <ahferroin7@...il.com>
Cc:	Andy Lutomirski <luto@...capital.net>,
	Harald Hoyer <harald@...hat.com>,
	Arnd Bergmann <arnd@...db.de>, Havoc Pennington <hp@...ox.com>,
	"linux-kernel\@vger.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

>>>>> "Austin" == Austin S Hemmelgarn <ahferroin7@...il.com> writes:

Austin> 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
>> 

Austin> Depending on how this is implemented, there would be a
Austin> potential issue if a message arrived for the client after the
Austin> daemon told it it could exit, but before it finished shutdown,
Austin> in which case the message might get lost.

What makes anyone think they can guarrantee that a message is even
received?  I could see the daemon sending the message and the client
getting a segfault and dumping core.  What then?  How would kdbus
solve this type of "race" anyway? 

Can anyone give a concrete example of one of the races that are closed
here?  That's been one of the missing examples.  And remember, there's
no perfection.  Even in the kernel we just had a discussion about
missed/missing IPIs and lost processor interrupts, etc.  Expecting
perfection is just asking for trouble.  

That's why there are timeouts, retries and just giving up and throwing
an exception.  

John
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