[libvirt] heisenbug in command.c

Serge Hallyn serge.hallyn at canonical.com
Fri Mar 16 17:10:37 UTC 2012


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.

>      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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