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SubjectRe: [PATCH] prctl: add PR_{SET,GET}_CHILD_SUBREAPER to allow simple process supervision
On 01/07, Valdis.Kletnieks@vt.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@poettering.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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