Re: [RFC PATCH] pidfd: implement PIDFD_THREAD flag for pidfd_open()

[Christian Brauner <brauner@kernel.org>]

On Wed, Jan 31, 2024 at 11:24:48AM -0800, Andy Lutomirski wrote:
> > On 01/31, Oleg Nesterov wrote:
> > >
> > > On 01/31, Andy Lutomirski wrote:
> > > Please note
> > >
> > >       /* TODO: respect PIDFD_THREAD */
> > >
> > > this patch adds into pidfd_send_signal().
> > >
> > > See also this part of discussion
> > >
> > >       > > +   /* TODO: respect PIDFD_THREAD */
> > >       >
> > >       > So I've been thinking about this at the end of last week. Do we need to
> > >       > give userspace a way to send a thread-group wide signal even when a
> > >       > PIDFD_THREAD pidfd is passed? Or should we just not worry about this
> > >       > right now and wait until someone needs this?
> > >
> > >       I don't know. I am fine either way, but I think this needs a separate
> > >       patch and another discussion in any case. Anyway should be trivial,
> > >       pidfd_send_signal() has the "flags" argument.
> > >
> > > with Christian in https://lore.kernel.org/all/20240130112126.GA26108@redhat.com/
> 
> I missed that.  Whoops.
> 
> On Wed, Jan 31, 2024 at 11:15 AM Oleg Nesterov <oleg@redhat.com> wrote:
> >
> > Forgot to mention...
> >
> > And I agree that pidfd_send_signal(flags => PGID/SID) can make
> > some sense too.
> >
> > But this a) doesn't depend on PIDFD_THREAD, and b) needs another
> > patch/discussion.
> >
> > But again, I am not sure I understood you correctly.
> >
> 
> Hmm.
> 
> When one works with regular (non-fd) pids / pgids etc, one specifies
> the signal domain at the time that one sends the signal.  I don't know
> what pidfds should do.  It seems a bit inefficient for anything that
> wants a pidfd and might send a signal in a different mode in the
> future to have to hold on to multiple pidfds, so it probably should be
> a pidfd_send_signal flag.
> 
> Which leaves the question of what the default should be.  Should
> pidfd_send_signal with flags = 0 on a PIDFD_THREAD signal the process
> or the thread?  I guess there are two reasonable solutions:
> 
> 1. flags = 0 always means process.  And maybe there's a special flag
> to send a signal that matches the pidfd type, or maybe not.
> 
> 2. flags = 0 does what the pidfd seems to imply, and a new
> PIDFD_SIGNAL_PID flag overrides it to signal the whole PID even if the
> pidfd is PIDFD_THREAD.
> 
> Do any of you have actual use cases in mind where one choice is
> clearly better than the other choice?

So conceptually I think having the type of pidfd dictate the default
scope of the signal is the most elegant approach. And then very likely
we should just have:

PIDFD_SIGNAL_THREAD
PIDFD_SIGNAL_THREAD_GROUP
PIDFD_SIGNAL_PROCESS_GROUP

I think for userspace it doesn't really matter as long as we clearly
document what's going on.

Thoughts?