[Devel] Re: [PATCH][usercr]: Ghost tasks must be detached

[Louis Rilling]

On 09/02/11  7:18 -0500, Oren Laadan wrote:
> 
> 
> On 02/09/2011 07:01 AM, Louis Rilling wrote:
> > On 08/02/11 18:09 -0800, Sukadev Bhattiprolu wrote:
> >> Oren Laadan [orenl at cs.columbia.edu] wrote:
> >> | 
> >> | 
> >> | On 02/05/2011 04:40 PM, Sukadev Bhattiprolu wrote:
> >> | > Oren Laadan [orenl at cs.columbia.edu] wrote:
> >> | > | Suka,
> >> | > | 
> >> | > | This patch - and the corresponding kernel patch - are wrong
> >> | > 
> >> | > Ah, I see that now.
> >> | > 
> >> | > But am not sure about the kernel part though. We were getting a crash
> >> | > reliably (with older kernels) because of the ->exit_signal = -1 in
> >> | > do_ghost_task().
> >> | 
> >> | Are we still getting it with 2.6.37 ?
> >>
> >> I am not currently getting the crash on 2.6.37 - I thought it was due to
> >> the following commit which removed the check for task_detached() in
> >> do_wait_thread().
> >>
> >> 	commit 9cd80bbb07fcd6d4d037fad4297496d3b132ac6b
> >> 	Author: Oleg Nesterov <oleg at redhat.com>
> >> 	Date:   Thu Dec 17 15:27:15 2009 -0800
> > 
> > I don't think that this introduced the bug. The bug triggers with EXIT_DEAD
> > tasks, for which wait() must ignore (see below). So, the bug looks still there
> > in 2.6.37.
> > 
> >>
> >> But if that is true, I need to investigate why Louis Rilling was getting
> >> the crash in Jun 2010 - which he tried to fix here:
> >>
> >> 	http://lkml.org/lkml/2010/6/16/295
> > 
> > I was getting the crash on Kerrighed, which heavily patches the 2.6.30 kernel.
> > I could reproduce it on vanilla Linux of the moment (2.6.35-rc3), but
> > only after introducing artificial delays in release_task().
> > 
> > IIRC, what triggers the crash is some exiting detached task in the
> > pid_namespace, which goes EXIT_DEAD, and as such cannot be reaped by
> > zap_pid_ns_processes()->sys_wait4(). So with some odd timing, the detached
> > task can call proc_flush_task() after container init does, which triggers the
> > proc_mnt crash.
> > 
> > Container init                           Some detached task in the ctnr
> >                                          exit_notify()
> > 						->exit_state = EXIT_DEAD
> > exit_notify()
> >  forget_original_parent()
> >   find_new_reaper()
> >    zap_pid_ns_processes()
> >     sys_wait4()
> >      /* cannot reap EXIT_DEAD tasks */
> >  /* reparents EXIT_DEAD tasks to global init */
> > 
> > Container reaper
> > release_task()
> >  proc_flush_task()
> >   pid_ns_release_proc()
> >                                          release_task()
> >                                           proc_flush_task()
> >                                            proc_flush_task_mnt()
> >                                             KABOOM
> 
> Louis, thanks for the explanation, and two follow-up questions:
> 
> 1) Is there a patch circulating for this ?  or even better, on the
> way to mainline ?

We finally agreed on a patch from Eric, but for some unknown reason, it has not
been finalized(?) and routed to mainline yet.

https://lkml.org/lkml/2010/7/12/213

> 
> 2) Would it suffice if the c/r code ensures that the init never
> exits before any EXIT_DEAD tasks ?

That's what Eric's patch does: make zap_pid_ns_processes() wait until all other
tasks (EXIT_DEAD or whatever) have passed
release_task()->__exit_signal()->__unhash_process().

Thanks,

Louis

-- 
Dr Louis Rilling			Kerlabs
Skype: louis.rilling			Batiment Germanium
Phone: (+33|0) 6 80 89 08 23		80 avenue des Buttes de Coesmes
http://www.kerlabs.com/			35700 Rennes
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