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

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Message-ID: <ZbU7d0dpTY08JgIl@tycho.pizza>
Date: Sat, 27 Jan 2024 10:20:55 -0700
From: Tycho Andersen <tycho@...ho.pizza>
To: Oleg Nesterov <oleg@...hat.com>
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 Sat, Jan 27, 2024 at 05:31:39PM +0100, Oleg Nesterov wrote:
> On 01/27, Tycho Andersen wrote:
> >
> > 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,

> > 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.

> > 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.

> > > 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