[systemd-devel] network/openvswitch dependency loop/deadlock

[Frederic Crozat]

Le jeudi 07 février 2013 à 09:55 -0600, Ian Pilcher a écrit :
> On 02/07/2013 06:13 AM, Colin Guthrie wrote:
> > 'Twas brillig, and Ian Pilcher at 06/02/13 22:27 did gyre and gimble:
> >> * systemd sees "Before=... network.target" in openvswitch.service and
> >>   waits for the network service to complete -- which will never happen,
> >>   because the network service is waiting for the openvswitch servicr to
> >>   start.
> >>
> >> * DEADLOCK!
> > 
> > This last step shouldn't (in theory) be a problem as far as I understand
> > it. Before=network.target doesn't imply it that has to wait for
> > network.service to complete - it should only imply that both
> > network.service and openvswitch.service are both have to start before
> > network.target is considered reached. If it said After=network.target
> > then I would see an obvious deadlock, but with both saying Before= they
> > should be able to work fine.
> 
> Well it would help if my brain worked a bit better.  openvswitch.service
> does, in fact, contain:
> 
>   After=syslog.target network.target
> 
> So there's the "obvious deadlock".
> 
> >> Assuming that the answer is yes, what is the best way to work around
> >> this?
> >>
> >> * Removing network.target from the Before=... line in
> >>   openvswitch.service is not an option.  See comment #1 of that bug.
> >>
> >> * Changing the network startup script (ifup-ovs) to use "systemctl
> >>   --ignore-dependencies start openvswitch.service" appears to work, but
> >>   the man page discourages its use for anything but debugging.
> > 
> > Depending on how the daemon is used, it might make more sense to use
> > --no-block. This will return control to the command line straight away,
> > but obviously the daemon may not be "ready" for communications yet and
> > the script may fail.
> 
> --no-block is not going to be an option.  The script is trying to bring
> up an Open vSwitch bridge, and it can't do so without a running daemon.
> 
> > I'm not familiar with the daemon or what it does and how any IPC may
> > work (i.e. how you talk to the daemon).
> 
> The script uses ovs-vsctl to talk to ovs-vswitchd.
> 
> > I get the feeling I'm perhaps misinterpreting something. I think the
> > real reason for the deadlock would be good to track down. It could be
> > that it is being artificially held back from completing or some other
> > dep is causing the problem.
> 
> You made the mistake of believing what I wrote.
> 
> > Also re the initscripts tweaks and the if statement proposed in the bug,
> > there is a SYSTEMCTL_IGNORE_DEPENDENCIES=1 env var you can export that
> > will make "service openvswitch start" pass the --ignore-dependencies
> > argument if it redirects to systemctl. That's likely cleaner than the if
> > [ -x /usr/bin/systemctl ] check. Obviously as this is arguably not the
> > right fix anyway, it's perhaps a moot point.
> 
> Cool.
> 
> So given that it is in fact "After=... network.target" (in
> openvswitch.service) that is causing the problem, do you see a better
> solution than using SYSTEMCTL_IGNORE_DEPENDENCIES?

We have exactly the same issue on openSUSE with some network services,
and we are using --ignore-dependencies in those case (because --no-block
is not always the "right" solution).

-- 
Frederic Crozat <fcrozat at suse.com>
SUSE