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

On Sat, Jan 27, 2024 at 05:31:39PM +0100, Oleg Nesterov wrote:

On 01/27, Tycho Andersen wrote:

quoted

It seems like (and the current pidfd_test enforces for some cases)

Which pidfd_test ?

I was thinking of poll_pidfd() in pidfd_test.c, but,

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we
want exactly one notification for a task dying.

This can't be right. EVERY user of poll_wait() or wait_event/etc
must handle/tolerate the false wakeups.

you're right, it doesn't enforce exactly once.

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I don't understand
how we guarantee this now, with all of these calls.

I don't understand why do we need or even want to guarantee this.

The extra wakeup must be always fine correctness-wise. Sure, it
would be nice to avoid the unnecessary wakeups, and perhaps we
can change wake_up_all() to pass a key to, say, only wake_up the
PIDFD_THREAD waiters from exit_notify(). But certainly this is
outside the scope of PIDFD_THREAD change we discuss.

The changes in do_notify_parent() (I have already sent the patch) and
in exit_notify() (proposed in my previous email) just ensure that,
with the minimal changes, we avoid 2 do_notify_pidfd's from the same
exit_notify() path.

Sounds good.

quoted

quoted

exit_notify() is called after exit_files(). pidfd_getfd() returns
ESRCH if the exiting thread completes release_task(), otherwise it
returns EBADF because ->files == NULL. This too doesn't really
depend on PIDFD_THREAD.

Yup, understood. It just seems like an inconsistency we might want to
fix.

Not sure this worth "fixing"...

Yep, maybe not. Just wanted to point it out.

Tycho