[Devel] Re: pspace child_reaper

[Cedric Le Goater]

Eric W. Biederman wrote:
> Cedric Le Goater <clg at fr.ibm.com> writes:
> 
>> Eric W. Biederman wrote:
>>> Cedric Le Goater <clg at fr.ibm.com> writes:
>>>
>>>> Hello All,
>>>>
>>>> Eric, in your initial proof of concept on the pid namespace, you were
>>>> defining a child_reaper per pid namespace.
>>>>
>>>> IMO, we can't use init_task as a child_reaper in a pid namespace because we
>>>> will have pid collision which might result in a breakage of the init_task.
>>> The kernel doesn't use init_task (The idle thread) once it starts
>>> init.  Reaping children is the job of pid == 1.
>> agree.
>>
>>>> Here are some questions on the model you intended to follow :
>>>>
>>>> Do you think we should have a child_reaper task per container ?
>>> We have an init per container so yes.
>> hmm, have we always ? what if i don't start an /sbin/init process in my
>> newly created pid namespace or container. where do I collect all the SIGCHLD ?
> 
> And this is the core question.
> 
>>>> Any completely different idea on the topic ?
>>> Init reaps the children, and I believe there are parts of user space
>>> that depend on this.  We shouldn't change that semantic.
>> IMHO, the only semantic i see is in the kernel, which needs someone to take
>> care of sigchld. /sbin/init is a very good candidate bc it collects sigchld
>> anyway and discards the ones it doesn't know about.
> 
> Roughly.  The other is a complete process tree.  Not having an init process will
> break the process tree.

I think there is some confusion here. we need a process 1 of course but it
does not have to be necessarily a user space /sbin/init process.

> I also believe that most of the advantages of not having an init
> process can be had with a trivial (probably static) init program that
> only calls waitpid.  Taking essentially no resources.

yes that could be one solution, or doing it in a kthread.

but leaving it up to user space to do the right thing (handle sigchld) in
the process doing the unshare seems a better solution.

C.
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