Re: xenbus and the message of doom

[Ian Campbell <Ian.Campbell@citrix.com>]

On Thu, 2011-12-15 at 19:20 +0000, Stefan Bader wrote:
> I was investigating a bug report[1] about newer kernels (>3.1) not booting as
> HVM guests on Amazon EC2. For some reason git bisect did give the some pain, but
> it lead me at least close and with some crash dump data I think I figured the
> problem.
> 
> commit ddacf5ef684a655abe2bb50c4b2a5b72ae0d5e05
> Author: Olaf Hering <olaf@aepfle.de>
> Date:   Thu Sep 22 16:14:49 2011 +0200
> 
>     xen/pv-on-hvm kexec: add xs_reset_watches to shutdown watches from old
>     kernel
> 
> This change introduced a xs_reset_watches() call. The problem seems to be that
> there is at least some version of Xen (I was able to reproduce with a 3.4.3
> version which I admit to deliberately not having updated) for which xenstore
> will not return any reply.
> 
> At least the backtraces in crash showed that xs_init had been calling
> xs_reset_watches() and that was happily idling in read_reply(). Effectively
> nothing was going on and the boot just hung.
> By just not doing that xs_reset_watches() call, I was able to boot under the
> same host. And for what it is worth there has not been an issue with Xen 4.1.1
> and a 3.0 dom0 kernel. Just this "older" release is trouble.

I sent a patch to fix exactly this issue in oxenstored (the ocaml
xenstore) just this week. Is there any chance that you are running C
xenstored with Xen 4.1.1 and oxenstored with Xen 3.4.3?

> Now the big question is, should this never happen and the host needs urgent
> updating. Or, should xs_talkv() set up a time limit and assume failure when not
> receiving a message after that? I could imagine the latter might lead at least
> to a more helpful "there is something wrong here, dude" than just hanging around
> without any response. ;)
> 
> -Stefan
> 
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