Race with hotplug vif scripts

Race with hotplug vif scripts

[Jacob Gorm Hansen]

hi,

I have a domU running a custom HTTP client. As soon as the domain is
booted and the VIF running it wants to initiate a connection. However, I
seem to be having a race where the domU starts using the VIF before it
is completely functional, probably because the hotplug script has not
completed.

Currently I just set the frontend to '4', and just wait for the backend
state to also become '4' (connected), but this does not work in all
cases. How can I synchronize with the hotplug script, and what is the
correct sequence of XenBus states?

Thanks,
Jacob

Re: Race with hotplug vif scripts

[Keir Fraser]

On 30/12/06 18:52, "Jacob Gorm Hansen" <jacobg@diku.dk> wrote:

> Currently I just set the frontend to '4', and just wait for the backend
> state to also become '4' (connected), but this does not work in all
> cases. How can I synchronize with the hotplug script, and what is the
> correct sequence of XenBus states?

Jumping straight to state 4 is fine. Unlike blkback, there is no interlock
between the hotplug scripts and netback. You could work around this by
having an interlock between your hotplug script and your domain builder. Or
we could add an interlock into netback, but this would probably have to
break compatibility between netback and net hotplug scripts.

 -- Keir

Re: Race with hotplug vif scripts

[Jacob Gorm Hansen]

On Sat, 2006-12-30 at 19:52 +0100, Jacob Gorm Hansen wrote:
> hi,
> 
> I have a domU running a custom HTTP client. As soon as the domain is
> booted and the VIF running it wants to initiate a connection. However, I
> seem to be having a race where the domU starts using the VIF before it
> is completely functional, probably because the hotplug script has not
> completed.
> 
> Currently I just set the frontend to '4', and just wait for the backend
> state to also become '4' (connected), but this does not work in all
> cases. How can I synchronize with the hotplug script, and what is the
> correct sequence of XenBus states?

Ahh never mind, the issue seems to be with TCP connection setup, the
server gets confused due to the shortlived domain reusing the same
source port number over and over. A little randomness fixes the problem.

Jacob

Re: Race with hotplug vif scripts

[Ewan Mellor]

On Sat, Dec 30, 2006 at 07:52:48PM +0100, Jacob Gorm Hansen wrote:

> hi,
> 
> I have a domU running a custom HTTP client. As soon as the domain is
> booted and the VIF running it wants to initiate a connection. However, I
> seem to be having a race where the domU starts using the VIF before it
> is completely functional, probably because the hotplug script has not
> completed.
> 
> Currently I just set the frontend to '4', and just wait for the backend
> state to also become '4' (connected), but this does not work in all
> cases. How can I synchronize with the hotplug script, and what is the
> correct sequence of XenBus states?

You can sync with the hotplug script using the store: the scripts write to
/local/domain/0/backend/<devclass>/<domid>/<devid>/hotplug-status, writing
either "error" or "connected" when they are done (see xen-hotplug-common.sh).
Xend uses this to make sure that the domain does not get unpaused until the
hotplugging is done, and your toolstack could do the same.

HTH,

Ewan.