Re: [PATCH v3 1/3] pidfd: allow pidfd_open() on non-thread-group leaders

On Thu, Jan 25, 2024 at 11:36:50AM -0700, Tycho Andersen wrote:

On Thu, Jan 25, 2024 at 07:30:46PM +0100, Oleg Nesterov wrote:

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On 01/25, Oleg Nesterov wrote:

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On 01/25, Tycho Andersen wrote:

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One of the things I don't like about PIDFD_THREAD is that it's hard to
tell whether an arbitrary thread is a leader or not. Right now we do
it by parsing /proc/pid/status, which shows all the stuff from
do_task_stat() that we don't care about but which is quite expensive
to compute. (Maybe there's a better way?)

With PIDFD_THREAD we could could do it twice, once with the flag, get
EINVAL, and then do it again. But ideally we wouldn't have to.

Too late for me, most probably I misunderstood.

If you want the PIDFD_THREAD behaviour, you can always use this flag
without any check...

Sorry, I hadn't read the patch. If it's ok to use PIDFD_THREAD on a
leader, then we can just always specify it. (We don't care about the
behavior of pidfd_poll().)

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Could you spell?

Just in case, we can even add PIDFD_AUTO (modulo naming) which acts as
PIDFD_THREAD if the target task is not a leader or 0 (current behaviour)
otherwise. Trivial.

Yep, or given the above, maybe it'll work as-is, thank you.

Yes, let's rather do the explicit PIDFD_THREAD.