Re: [libvirt] heisenbug in command.c
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? That
seems very much unrelated.
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).
-jim
> ret = virPidWait(cmd->pid, exitstatus ? exitstatus
:&status);
> if (ret == 0) {
> cmd->pid = -1;
> cmd->reap = false;
> if (status) {
> char *str = virCommandToString(cmd);
> char *st = virCommandTranslateStatus(status);
> virCommandError(VIR_ERR_INTERNAL_ERROR,
> _("Child process (%s) status unexpected:
%s"),
> str ? str : cmd->args[0], NULLSTR(st));
>
>> But it's only been seen on two (very
>>different) machines, and the slightest shifting of the winds makes it go
>>away. Given how sneaky this bug appears to be, there's a slight
>>temptation to have iptablesAddRemoveRule pass in a int* for status and
>>better deal with the -EINTR. But I fear that might be papering over a
>>worse race.
>
>I don't follow how you think there is a -EINTR being encountered in
>libvirt.
Yeah I don't really either.
> I think you'd be better off investigating why iptables really
>is exiting with status 4.
Well, given what EINTR means, shouldn't src/util/iptables.c re-try
the command if it gets that?
Anyway I'll keep digging, but was wondering if anyone else has seen this.
-serge
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