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Re: [libvirt] heisenbug in command.c



On 03/16/2012 03:52 PM, Jim Paris wrote:
Serge Hallyn wrote:
On 03/16/2012 11:50 AM, Eric Blake wrote:
On 03/16/2012 10:36 AM, Serge Hallyn wrote:
Hi,

It seems I've run into quite the heisenbug, reported at
https://bugs.launchpad.net/ubuntu/+source/libvirt/+bug/922628

It manifests itself as virPidWait returning status=4 for iptables (which
should never exit with status=4).

Maybe iptables isn't documented as exiting with $? of 4, but that's what
is happening.  The libvirt code in question is quite clear that it
grabbed an accurate exit status from the child process.


Well, yes.  I figured that either (1) iptables actually got -EINTR
from the kernel and passed that along as its exit code, or (2)
something went wrong with memory being overwritten in libvirt,
however unlikely. Stranger things have happened.  If (1), I was
wondering if it was being ignored on purpose.

Why do you bring up EINTR at all?  Just because EINTR is 4?

Yup.

>  That
seems very much unrelated.

I didn't think so, but looks like you're right :)

This is from iptables:

enum xtables_exittype {
         OTHER_PROBLEM = 1,
         PARAMETER_PROBLEM,
         VERSION_PROBLEM,
         RESOURCE_PROBLEM,
         XTF_ONLY_ONCE,
         XTF_NO_INVERT,
         XTF_BAD_VALUE,
         XTF_ONE_ACTION,
};

So it looks like iptables is returning RESOURCE_PROBLEM (which could
explain why it's intermittent).

That makes a lot more sense then, yes. When scripting a bunch of parallel iptables rules adds (followed by waits), I do once in awhile get

iptables: Resource temporarily unavailable.

(and sometimes a -EINVAL message)

though 'wait' still says status was 0. I've never gotten 4. The next run then succeeds.

This still however sounds like src/util/iptables.c might ought to re-run the command if it gets 4.

thanks,
-serge


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