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

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Message-ID: <20240123222231.GA25162@redhat.com>
Date: Tue, 23 Jan 2024 23:22:31 +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>
Subject: Re: [PATCH v3 1/3] pidfd: allow pidfd_open() on non-thread-group
 leaders

I am already sleeping. I'll try to reply to other parts of your email
tomorrow but I am not sure, I will be very busy with family duties.

On 01/23, Tycho Andersen wrote:
>
> > >  	__exit_signal(p);
> >
> > and,  do_notify_pidfd() is called before __exit_signal() which does
> > __unhash_process() -> detach_pid(PIDTYPE_PID).
> >
> > Doesn't this mean that pidfd_poll() can hang? thread_group_exited()
> > won't return true after do_notify_pidfd() above, not to mention that
> > thread_group_empty() is not possible if !thread_group_leader().
>
> I was wondering about this too, but the test_non_tgl_poll_exit test in
> the next patch tests exactly this and works as expected.

Well, if release_task() completes __exit_signal() before the woken task
does thread_group_exited(), pid_task(PIDTYPE_PID) will return 0 and
pidfd_poll() won't hang.

But to be honest I can't understand test_non_tgl_poll_exit() at all. I don't
even understand why the process/thread created by fork_task_with_thread()
should ever exit. And why it creates the "writer" child... Never mind, too
late for me to read the code.

Oleg.