[Devel] Re: [PATCH 1/15] Move exit_task_namespaces()

[Oleg Nesterov]

On 08/06, Pavel Emelyanov wrote:
>
> Oleg Nesterov wrote:
> >On 08/06, Pavel Emelyanov wrote:
> >>Oleg Nesterov wrote:
> >>>On 07/26, Pavel Emelyanov wrote:
> >>>>The reason to release namespaces after reparenting is that when task
> >>>>exits it may send a signal to its parent (SIGCHLD), but if the parent
> >>>>has already exited its namespaces there will be no way to decide what
> >>>>pid to dever to him - parent can be from different namespace.
> >>>I almost forgot about this one...
> >>>
> >>>After reading the whole series, I can't understand the above explanation
> >>>any longer. The parent can't be from different namespace, either we have
> >>>another sub-thread, or we reparent the child to /sbin/init which should
> >>>be from the same namespace.
> >>If the child that is a new namespace's init is exiting its parent is from 
> >>the
> >>different namespace.
> >
> >In that case it doesn't have childs. The were SIGKILL'ed before 
> >exit_notify().
> 
> It does not, but it's parent - does :)

Yes. But in that case forget_original_parent() is no-op! This means that
it is not needed to move exit_task_namepsace() after.

> >>Moreover, we will probably want to implement "entering"
> >>the pid namespace, so  having tasks with parents from another namespace 
> >>will
> >>be OK.
> >
> >Well. I saw this word "entering", but I don't know the meaning. Just 
> >curious,
> >could you explain?
> 
> "Entering" means "moving task to arbitrary namespace"

Heh. Very much nontrivial, good luck :) At least we need to grow ->numbers[],
and if its pid was pinned...

> >And, if an exiting task has a child which is already from another 
> >namespace,
> >why can't we release our namespace before re-parenting? I guess I need to
> >know what "entering" means to understand this...
> 
> One of the desired actions was the following:
> 1. task X clones the new namespace with the child Y as this namespace's 
> init;
> 2. task X waits for SIGCHILD to come informing that the namespace is dead.
> In this scenario we need to set the Y's pid as it is seen from X's 
> namespace in siginfo.

Yes sure. But again, how this is connected to "we should do exit_task_namespace()
after forget_original_parent()" ?

I guess I missed something stupid and simple...

Oleg.