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

[Oren Laadan]

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 ?
> 
> One fix I was watching for was Eric Biederman's 
> 
> 	http://lkml.org/lkml/2010/7/12/213
> 
> which AFAICT has not been merged yet.

If we need it and it isn't in mainline (any reason why ?) then
we can just add it to our linux-cr tree, as a preparatory patch.

> 
> Was there another change to 2.6.37 that would prevent the crash ?

I don't know whether *that* crash still happens in 2.6.37 - 
because I still didn't test it with that kernel line back.
(Actually, I never experienced that crash here even with
earlier kernels).

> 
> | (I should have noticed it in the review!). It turns out that
> | ghost (and dead) tasks are _not_ auto-reaped anymore.
> | 
> | There are only two way for tasks to be auto-reaped: one is if
> | their parent explicitly says so in its sighand information (but
> | then it applies to all children). The other way is if they have
> | ->exit_signal==-1. From userspace this happens only when cloning 
> | with CLONE_THREAD. Using 0xFF for the @flags argument to clone()
> | syscall instead results in ->exit_signal = 255 ...
> | 
> | The original motivation for this patch was:
> | 
> | > The downside of marking the task detached in do_ghost_task() is that
> | > with current/older kernels container-init does not wait for detached
> | > tasks.  See:
> | > 
> | > 	http://lkml.org/lkml/2010/6/16/272
> | > 	http://lkml.org/lkml/2010/7/12/213
> | > 
> | > This can lead to a kernel crash if the container-init pre-deceases a
> | > ghost task.
> | 
> | Is this still a problem in 2.6.37 ?
> 
> Well, some inadvertent userspace changes seemed to cause the crash (or
> an application hang on some machines) during restart. By making those changes,
> I seem to be getting an application hang 5 out of 6 times even with 2.6.37,
> but did not get a crash. I will investigate this new hang next week.

I'm currently chasing down a bug that causes restart to hang when
there are ghost/dead tasks. It may be the same one you are seeing.
So far I'm convinced it's userspace - working on it. Will post 
patches once solved.

Thanks,

Oren.


> 
> | 
> | Oren.
> | 
> | 
> | On 01/10/2011 08:51 PM, Oren Laadan wrote:
> | > 
> | > Applied to user-cr.
> | > 
> | > Thanks,
> | > 
> | > Oren.
> | > 
> | > On 12/10/2010 10:35 PM, Sukadev Bhattiprolu wrote:
> | >>
> | >> From: Sukadev Bhattiprolu <sukadev at linux.vnet.ibm.com>
> | >> Date: Fri, 10 Dec 2010 19:23:58 -0800
> | >> Subject: [PATCH 1/1] Ghost tasks must be detached
> | >>
> | >> Ghost processes are created only to help restore orphaned sessions/pgrps.
> | >> As such once the session/pgrp is created the ghost must not send another
> | >> SIGCHLD to the parent but exit silently.  So create such tasks as
> | >> "detached".
> | >>
> | >> See also:
> | >>
> | >> https://lists.linux-foundation.org/pipermail/containers/2010-December/026076.html
> | >>
> | >> Signed-off-by: Sukadev Bhattiprolu <sukadev at linux.vnet.ibm.com>
> | >> ---
> | >>  restart.c |    7 +++++++
> | >>  1 files changed, 7 insertions(+), 0 deletions(-)
> | >>
> | >> diff --git a/restart.c b/restart.c
> | >> index 9fb5e9f..d7ba26b 100644
> | >> --- a/restart.c
> | >> +++ b/restart.c
> | >> @@ -1744,6 +1744,13 @@ static pid_t ckpt_fork_child(struct ckpt_ctx *ctx, struct task *child)
> | >>  		flags |= CLONE_THREAD | CLONE_SIGHAND | CLONE_VM;
> | >>  	else if (child->flags & TASK_SIBLING)
> | >>  		flags |= CLONE_PARENT;
> | >> +	else if (child->flags & (TASK_GHOST|TASK_DEAD)) {
> | >> +		/* 
> | >> +		 * Ghosts must vanish silently (without signalling parent)
> | >> +		 * when they are done.
> | >> +		 */
> | >> +		flags = 0xFF;
> | >> +	}
> | >>  
> | >>  	memset(&clone_args, 0, sizeof(clone_args));
> | >>  	clone_args.nr_pids = 1;
> | > _______________________________________________
> | > Containers mailing list
> | > Containers at lists.linux-foundation.org
> | > https://lists.linux-foundation.org/mailman/listinfo/containers
> | > 
> 
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