[Devel] Re: [PATCH][cr]: Ghost tasks must not be detached in kernel

[Oren Laadan]

Merged into v23-rc1.

Thanks,

Oren.

On 12/10/2010 10:33 PM, Sukadev Bhattiprolu wrote:
> Oren,
> 
> As discussed (a while ago)
> 
> https://lists.linux-foundation.org/pipermail/containers/2010-November/025905.html
> 
> here is a patch. I will send a follow-on user-cr patch to mark
> the task detached in user-space.
> 
> ---
> 
> From: Sukadev Bhattiprolu <sukadev at linux.vnet.ibm.com>
> Date: Fri, 10 Dec 2010 18:37:17 -0800
> Subject: [PATCH 1/1] Ghost tasks must not be detached in kernel
> 
> Ghost tasks are "temporary" tasks created during sys_restart() to
> restore "orphaned" sessions/process groups (i.e. those in which the
> original session/pgrp leader had exited before checkpoint but other
> processes exist in that session/pgrp).
> 
> We only create the ghost task so we have a valid session/pgrp that
> the other processes in the session/pgrp can use. Once those other
> processes are created, the ghost tasks must silently vanish (without
> sending another SIGCHLD).
> 
> do_ghost_task() was setting the exit_signal to -1 so the ghosts don't
> send a signal. But rather than set the ->exit_signal here, the caller
> (i.e user-space) can create the ghost task by explicitly setting the
> CSIGNAL mask to 0xFF. i.e it is not something the we need to do in the
> kernel.
> 
> 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.
> 
> Signed-off-by: Sukadev Bhattiprolu (sukadev at us.ibm.com)
> ---
>  kernel/checkpoint/restart.c |    1 -
>  1 files changed, 0 insertions(+), 1 deletions(-)
> 
> diff --git a/kernel/checkpoint/restart.c b/kernel/checkpoint/restart.c
> index 17270b8..2bb7197 100644
> --- a/kernel/checkpoint/restart.c
> +++ b/kernel/checkpoint/restart.c
> @@ -972,7 +972,6 @@ static int do_ghost_task(void)
>  	if (ret < 0)
>  		ckpt_err(ctx, ret, "ghost restart failed\n");
>  
> -	current->exit_signal = -1;
>  	restore_debug_exit(ctx);
>  	ckpt_ctx_put(ctx);
>  	do_exit(0);
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