linux-kernel - Re: [PATCH 1/3] pidfd_poll: report POLLHUP when pid

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Date: Fri, 2 Feb 2024 17:07:05 +0100
From: Oleg Nesterov <oleg@...hat.com>
To: Christian Brauner <brauner@...nel.org>
Cc: Andy Lutomirski <luto@...capital.net>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Tycho Andersen <tycho@...ho.pizza>, linux-api@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/3] pidfd_poll: report POLLHUP when pid_task() == NULL

On 02/02, Christian Brauner wrote:
>
> > TODO: change do_notify_pidfd() to use the keyed wakeups.
>
> How does the following appended patch look?

No, no.

I think we need a simpler patch. I was going to send it as 4/4, but I'd
like to think more, _perhaps_ we can also discriminate the PIDFD_THREAD
and non-PIDFD_THREAD waiters. I'll try to make the patch(es) tomorrow or
at least provided more info.

3 notes for now:

	1. we can't use wake_up_poll(), it passes nr_exclusive => 1

	2. exit_notify() should not pass EPOLLHUP to wake_up, we do
	   not want to wake up the { .events = POLLHUP } waiters.

	3. we do not need to change __change_pid().

	   Well, _perhaps_ it can/should use __wake_up_pollfree(), but
	   I need to check if fs/select.c use "autoremove" or not.


> -static __poll_t pidfd_poll(struct file *file, struct poll_table_struct *pts)
> +static __poll_t pidfd_poll(struct file *file, poll_table *wait)
>  {
>  	struct pid *pid = file->private_data;
>  	bool thread = file->f_flags & PIDFD_THREAD;
>  	struct task_struct *task;
>  	__poll_t poll_flags = 0;
>  
> -	poll_wait(file, &pid->wait_pidfd, pts);
> +	poll_wait(file, &pid->wait_pidfd, wait);

This is correct but only cosemtic and has nothing to do with what
we discuss?

Oleg.