[CRIU] hang when restoring container with zombies

[Tycho Andersen]

On Mon, Jul 20, 2015 at 03:40:36PM +0300, Pavel Emelyanov wrote:
> On 07/20/2015 03:16 PM, Andrew Vagin wrote:
> > On Fri, Jul 17, 2015 at 09:20:28PM +0300, Pavel Emelyanov wrote:
> >> On 07/17/2015 07:41 PM, Tycho Andersen wrote:
> >>> On Fri, Jul 17, 2015 at 07:15:55PM +0300, Pavel Emelyanov wrote:
> >>>> On 07/17/2015 06:36 PM, Tycho Andersen wrote:
> >>>>> Hi all,
> >>>>>
> >>>>> I'm experiencing a hang when restoring a process with zombies; the
> >>>>> zombies exit, but the parent process (in this case the container's
> >>>>> init) isn't getting the SIGCHLD, so it just gets stuck waiting for
> >>>>> zombies_inprogress. The parent process' /proc/pid/status is below, and
> >>>>> it doesn't seem to be blocking SIGCHLD and there are no pending
> >>>>> signals. I stuck a printf in the sigchld_handler in the restorer blob,
> >>>>> and it does get called for sid helpers, but not for the zombie
> >>>>> processes.
> >>>>>
> >>>>> Does anyone have any ideas about what's going wrong? I have no idea
> >>>>> why the signal would be blocked.
> >>>>
> >>>> Maybe it's not blocked but merged with another (previous) sigchild?
> >>>
> >>> Yep, I just traced it and that's exactly what's happening. So I think
> >>> the right thing to do here is to waitpid() in a loop in the restorer
> >>> blob's sigchld_handler so that we make sure to collect all the
> >>> processes that have died?
> >>
> >> No, we can't call the waitpid() in pie/restore.c's sigchil_handler()
> >> since we must _leave_ the zombie in zombie state :)
> > 
> > We can try to use waitid with WNOWAIT
> > """
> > WNOWAIT     Leave the child in a waitable state; a later wait call  can
> > 	   be used to again retrieve the child status information.
> > """
> 
> This would mean that we rework the existing zombie wait logic. I'm OK with
> it, but would appreciate if we find out what's wrong with the existing one :)

The problem is that it's getting coalesced with a helper task's
SIGCHLD, so if the helper exits first and then the zombie, we miss the
zombie's exit.

I'll try to send a patch with WNOWAIT.

Tycho