Re: [Xen-devel] xenbus and the message of doom

[Konrad Rzeszutek Wilk <konrad@xxxxxxxxxx>]

On Thu, Dec 15, 2011 at 08:20:23PM +0100, 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.

Stefan, thanks for finding this.

Olaf, what are your thoughts? Should I prep a patch to revert the patch
below and then we can work on 3.3 and rethink this in 3.3? The clock is
ticking for 3.2 and there is not much runway to fix stuff.

> 
> commit ddacf5ef684a655abe2bb50c4b2a5b72ae0d5e05
> Author: Olaf Hering <olaf@xxxxxxxxx>
> 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.

And oxenstore too, but Ian prepped a patch for this. Perhaps that is
what Amazon is running.
> 
> 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.

So at least we should have a timeout read_reply. But I don't see
anything in the code that we could immediately use.

> 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.
> 
> 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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