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

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Date: Sat, 27 Jan 2024 17:31:39 +0100
From: Oleg Nesterov <oleg@...hat.com>
To: Tycho Andersen <tycho@...ho.pizza>
Cc: Christian Brauner <brauner@...nel.org>, linux-kernel@...r.kernel.org,
	linux-api@...r.kernel.org, Tycho Andersen <tandersen@...flix.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>
Subject: Re: [PATCH v3 1/3] pidfd: allow pidfd_open() on non-thread-group
 leaders

On 01/27, Tycho Andersen wrote:
>
> It seems like (and the current pidfd_test enforces for some cases)

Which pidfd_test ?

> 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.

> 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.

> > 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"...

Oleg.