Re: [Xen-devel] [PATCH 1/2] tools: remove systemd xenstore socket definitions

[Juergen Gross <jgross@xxxxxxxx>]

On 20/07/16 12:52, George Dunlap wrote:
> On Wed, Jul 20, 2016 at 10:58 AM, Juergen Gross <jgross@xxxxxxxx> wrote:
>> On 20/07/16 11:02, Ross Lagerwall wrote:
>>> On 06/29/2016 02:44 PM, Juergen Gross wrote:
>>>> On 29/06/16 15:31, Ross Lagerwall wrote:
>>>>> On 06/29/2016 02:00 PM, Juergen Gross wrote:
>>>>>> On 29/06/16 14:52, Andrew Cooper wrote:
>>>>>>> On 29/06/16 13:44, Juergen Gross wrote:
>>>>>>>> @@ -2068,13 +1964,6 @@ int main(int argc, char *argv[])
>>>>>>>>      /* Tell the kernel we're up and running. */
>>>>>>>>      xenbus_notify_running();
>>>>>>>>
>>>>>>>> -#if defined(XEN_SYSTEMD_ENABLED)
>>>>>>>> -    if (systemd) {
>>>>>>>> -        sd_notify(1, "READY=1");
>>>>>>>> -        fprintf(stderr, SD_NOTICE "xenstored is ready\n");
>>>>>>>> -    }
>>>>>>>> -#endif
>>>>>>>
>>>>>>> Getting rid of the socket configuration for systemd is ok, but we
>>>>>>> should
>>>>>>> keep the sd_notify() calls for when the daemon is started by systemd.
>>>>>>>
>>>>>>> Socket activiation and sd_notify() are orthogonal, and sd_notify() is
>>>>>>> still required if we don't want systemd to treat xenstored as a legacy
>>>>>>> unix daemon.
>>>>>>
>>>>>> So what is the downside of xenstored being treated as a legacy daemon?
>>>>>> This question is especially interesting for the case of patch 2 being
>>>>>> considered: xenstored is no longer started by systemd, but by a wrapper
>>>>>> script which might decide to start the xenstore domain instead.
>>>>>>
>>>>>> Another problem: today xenstored decides whether to call sd_notify()
>>>>>> by testing the xenstore sockets being specified via systemd. This will
>>>>>> no longer work. So how to do it now?
>>>>>>
>>>>>>
>>>>>
>>>>> One problem with the patch as it currently is implemented is that the
>>>>> service type is not correct for when xenstored is a daemon. This makes
>>>>> it difficult to manage with systemd and difficult for other services to
>>>>> depend on it in a sensible fashion. The end result is subtle races and
>>>>> occasional failures.
>>>>
>>>> Could you please educate me what's wrong? I'm no systemd expert.
>>>
>>> Sorry for the late response.
>>>
>>> With Type=oneshot, systemd considers the service to be "started" as soon
>>> as the ExecStart command completes. After re-reading the patches, I
>>> realize that timeout_xenstore() should ensure that xenstored is actually
>>> ready before systemd starts dependent services. This should prevent the
>>> races I was mentioned previously.
>>>
>>> Nevertheless, I feel that this patch series makes the implementation
>>> worse for users of xenstored as a daemon:
>>>
>>> - Because Type=oneshot is not intended to be used for daemon processes,
>>> systemctl status does not show the service as "running", instead it
>>> shows "exited". This is surprising, at the very least. I haven't tested,
>>> but I believe it would also prevent us someday using the systemd service
>>> management features like Restart=on-failure
>>>
>>> - Removes socket activation. Services would have to explicitly depend on
>>> xenstored.service rather than having an implicit dependency on simply by
>>> using xenstored.socket. This means configuring explicit ordering, etc.,
>>> which is easier to get wrong. Socket activation is also generally the
>>> most efficient way of starting a service.
>>
>> So you are not taking xenstore domain into account at all.
>>
>> There is no socket for xenstore domain so wanting to rely on socket
>> activation is the wrong way.
>>
>>> - Uses a poll loop to determine whether the daemon is ready or not
>>> rather than explicit notification from the daemon itself.
>>
>> How to do this in the domain case?
>>
>>> - Uses a shell script wrapper...
>>
>> That's on purpose: this enables me to switch between both setups
>> (daemon or domain) by modifying only _one_ configuration file.
> 
> FWIW I'm pretty sure using a shell script wrapper also clobbers the
> SELinux context.  (I forget whether this means xenstore doesn't run or
> xenstore runs completely unrestricted.)

Okay, this would need to be corrected. I assume it is possible to
address such an issue in a script?

> (Ross, does XenServer use SELinux?  Any ideas here?)
> 
>>> I would suggest asking on the systemd-devel mailing list or the #systemd
>>> IRC channel on freenode. They should be able to suggest the best way of
>>> solving this.
>>
>> I'm sure I could tweak the whole setup more in direction of systemd.
>> I'm not sure the final picture would be the preferred one.
> 
> But as Ross said, there are advantages to using the systemd-specific
> functionality; and it's likely that the majority of our users will be
> running xenstored in dom0.  Doesn't it make sense to at least see if
> there are sensible options for how we can take advantage of those
> before we just drop it?

I still don't see the advantages, as relying on socket activation of
the xenstore service is no option: as soon as you do this you drop the
possibility to ever use the xenstore domain on such a system.

To be clear: I don't want to avoid systemd by any means. I just don't
want to have a complex and ugly solution with no gain just because
doing it the systemd way.


Juergen

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