linux-kernel - Re: [PATCH] prctl: add PR_{SET,GET}_CHILD_SUBREAPER to allow simple process supervision

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Date:	Mon, 9 Jan 2012 16:07:01 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Valdis.Kletnieks@...edu
Cc:	Kay Sievers <kay.sievers@...y.org>, akpm@...ux-foundation.org,
	linux-kernel@...r.kernel.org,
	Lennart Poettering <lennart@...ttering.net>
Subject: Re: [PATCH] prctl: add PR_{SET,GET}_CHILD_SUBREAPER to allow
	simple process supervision

On 01/07, Valdis.Kletnieks@...edu wrote:
>
> On Sat, 07 Jan 2012 16:56:37 +0100, Kay Sievers said:
> > Resending this, it got lost last year's September.
> >
> > We still need it to properly implement init-like service managers.
>
> > From: Lennart Poettering <lennart@...ttering.net>
> > Subject: prctl: add PR_{SET,GET}_CHILD_SUBREAPER to allow simple process supervision
>
> > Users of this will be the systemd per-user instance, which provides
> > init-like functionality for the user's login session and D-Bus, which
> > activates bus services on-demand. Both need init-like capabilities
> > to be able to properly keep track of the services they start.
>
> > --- a/include/linux/sched.h
> > +++ b/include/linux/sched.h
> > @@ -552,6 +552,18 @@ struct signal_struct {
> >  	int			group_stop_count;
> >  	unsigned int		flags; /* see SIGNAL_* flags below */
> >
> > +	/*
> > +	 * PR_SET_CHILD_SUBREAPER marks a process, like a service
> > +	 * manager, to re-parent orphan (double-forking) child processes
> > +	 * to this process instead of 'init'. The service manager is
> > +	 * able to receive SIGCHLD signals and is able to investigate
> > +	 * the process until it calls wait(). All children of this
> > +	 * process will inherit a flag if they should look for a
> > +	 * child_subreaper process at exit.
> > +	 */
> > +	unsigned int		is_child_subreaper:1;
> > +	unsigned int		has_child_subreaper:1;
>
> Is there someplace we can stick these two fields where they won't expand the
> signal_struct?  Can we stick them in signal_struct->flags instead?

Yes, it would be better to use signal_struct->flags. But we can't do this
until we cleanup the usage of ->flags. For example, task_participate_group_stop
simply does sig->flags = SIGNAL_STOP_STOPPED.


> > +		/* find the first ancestor marked as child_subreaper */
> > +		for (reaper = father->real_parent;
> > +		     reaper != &init_task;
> > +		     reaper = reaper->real_parent) {
>
> I admit being insufficiently caffienated - does this DTRT in a PID namespace? That
> &init_task looks fishy to me...

Probably this needs a comment. Initially I was confused too.

Note that the code below checks same_thread_group(reaper, pid_ns->child_reaper),
this is what we need to DTRT in a PID namespace. However we still need the
check above, see http://marc.info/?l=linux-kernel&m=131385460420380

Oleg.

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