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

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Date:	Tue, 16 Aug 2011 17:45:20 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Kay Sievers <kay.sievers@...y.org>
Cc:	linux-kernel@...r.kernel.org,
	Lennart Poettering <lennart@...ttering.net>
Subject: Re: [PATCH] prctl: add PR_{SET,GET}_CHILD_REAPER to allow simple
 process supervision

On Wed, 17 Aug 2011 02:32:39 +0200 Kay Sievers <kay.sievers@...y.org> wrote:

> On Tue, Aug 16, 2011 at 22:10, Andrew Morton <akpm@...ux-foundation.org> wrote:
> > On Fri, 29 Jul 2011 02:01:44 +0200
> > Kay Sievers <kay.sievers@...y.org> wrote:
> >
> >> From: Lennart Poettering <lennart@...ttering.net>
> >> Subject: prctl: add PR_{SET,GET}_CHILD_REAPER to allow simple process supervision
> >>
> >> Userspace service managers/supervisors need to track their started
> >> services. Many services daemonize by double-forking and get implicitely
> >> re-parented to PID 1. The process manager will no longer be able to
> >> receive the SIGCHLD signals for them.
> >>
> >> With this prctl, a service manager can mark itself as a sort of
> >> 'sub-init' process, able to stay as the parent process for all processes
> >> created by the started services. All SIGCHLD signals will be delivered
> >> to the service manager.
> >>
> >> As a side effect, the relevant parent PID information does not get lost
> >> by a double-fork, which results in a more elaborate process tree and 'ps'
> >> output.
> >>
> >> This is orthogonal to PID namespaces. PID namespaces are isolated
> >> from each other, while a service management process usually requires
> >> the serices to live in the same namespace, to be able to talk to each
> >> other.
> >>
> >> 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 on-demand. Both will need init-like capabilities
> >> to be able to properly keep track of the services they start.
> >>
> >
> > Interesting patch. __I can't immediately see any nasty effects from it..
> >
> > Did you consider using the existing taskstats capability for this?
> 
> Yes, but as it always is with buffered async interfaces, they are
> tricky regarding ordering, races and possible overflows.
> 
> SIGCHLD is async too, but it has important differences in this case:
> If the service-manager is the reaper, it will do the waitpid() itself,
> and before it reaps the child, it can still investigate the existing
> task and it will also directly receive the return values from
> waitpid(). If we let the pids re-parent to PID 1, then the dead pids
> and most of their information is gone before the service manager sees
> the taskstats event.
> 
> The service-manager needs to handle SIGCHLD and waitpid() anyway for
> all the stuff that does not double-fork, so the code is already there
> and does all what we need without involving a second interface just
> for re-parenting processes.
> 
> My very personal favourite is that 'ps afx' looks so nice now. The
> tree of the processes of the login session start to make sense, and we
> don't have half of the user processes hanging off PID 1. But that's
> surely just cosmetics, and no reason to do that. I just like pretty
> things. :)

Spose so.  I spy suitable changelog enhancements.

Also, other means of notification if they exist.  I'm sure they do ;)

> > The comment block over find_new_reaper() is now incomplete. __Please
> > update it?
> 
> '... give it to the child reaper process (ie "init") in out pid
> space.' still kind of fits, I think?
> 
> Would:
> '... give it to the child reaper process (ie 'init' or parent marked
> as reaper) in our pid space.' sound better?

At a minimum.  A nice discourse on what that code is doing in there
(and why!) would be better.  After all, the comment is supposed to
explain the function.

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