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

[Oren Laadan <orenl-eQaUEPhvms7ENvBUuze7eA@public.gmane.org>]

Suka,

This patch - and the corresponding kernel patch - are wrong
(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 ?

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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