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

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Date: Fri, 2 Feb 2024 20:50:32 +0100
From: Christian Brauner <brauner@...nel.org>
To: Oleg Nesterov <oleg@...hat.com>
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 Fri, Feb 02, 2024 at 08:05:29PM +0100, Oleg Nesterov wrote:
> On 02/02, Christian Brauner wrote:
> >
> > > 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
> >
> > Right, I didn't go that far.
> >
> > > at least provided more info.
> > >
> > > 3 notes for now:
> > >
> > > 	1. we can't use wake_up_poll(), it passes nr_exclusive => 1
> >
> > Bah. So we need the same stuff we did for io_uring and use
> > __wake_up() directly. Or we add wake_up_all_poll() and convert the other
> > three callsites:
> 
> ...
> 
> > +#define wake_up_all_poll(x, m)                                                 \
> > +       __wake_up(x, TASK_NORMAL, 0, poll_to_key(m))
> 
> Agreed, but I think this + s/wake_up/wake_up_all_poll/ conversions
> need a separate patch.

Yeah, I know. This is just a scribbled draft.

> 
> 
> > -void do_notify_pidfd(struct task_struct *task)
> > +void pidfd_wake_up_poll(struct task_struct *task, bool dead)
> >  {
> > -	struct pid *pid;
> > -
> >  	WARN_ON(task->exit_state == 0);
> > -	pid = task_pid(task);
> > -	wake_up_all(&pid->wait_pidfd);
> > +	WARN_ON(mask == 0);
> > +	wake_up_all_poll(&task_pid(task)->wait_pidfd,
> > +			 EPOLLIN | EPOLLRDNORM | dead ? EPOLLHUP : 0);
> 
> No...
> 
> This is still overcomplicated and is not right.

I'm all ears.

> Christian, I'll write another email tomorrow.

Sure, there's no rush. I had not intended that this patch be used. I
have another large series I need to take care of so I can't spend a lot
of time on writing this anyway. I just hadn't used the keyed apis before
and got curious. So don't get the impression that I intend to write
this. I fully expected you to do it.