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

[Andy Lutomirski]

> 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@xxxxxxxxxx/

I missed that.  Whoops.

On Wed, Jan 31, 2024 at 11:15 AM Oleg Nesterov <oleg@xxxxxxxxxx> 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?

--Andy