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

[Sukadev Bhattiprolu <sukadev-23VcF4HTsmIX0ybBhKVfKdBPR1lH4CV8@public.gmane.org>]

Oren Laadan [orenl-eQaUEPhvms7ENvBUuze7eA@public.gmane.org] 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().

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.

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

| (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.

| 
| 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-23VcF4HTsmIX0ybBhKVfKdBPR1lH4CV8@public.gmane.org>
| >> 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-23VcF4HTsmIX0ybBhKVfKdBPR1lH4CV8@public.gmane.org>
| >> ---
| >>  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;
| > _______________________________________________
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| >