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Date: Wed, 31 Jan 2024 17:10:53 +0100
From: Oleg Nesterov <oleg@redhat.com>
To: Christian Brauner <brauner@kernel.org>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>,
	Tycho Andersen <tycho@tycho.pizza>, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v3 0/1] pidfd: implement PIDFD_THREAD flag for
 pidfd_open()
Message-ID: <20240131161053.GC2609@redhat.com>
References: <20240131132541.GA23632@redhat.com>
 <20240131141204.GA24130@redhat.com>
 <20240131-engel-entern-9b5c96659948@brauner>
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On 01/31, Christian Brauner wrote:
>
> On Wed, Jan 31, 2024 at 03:12:04PM +0100, Oleg Nesterov wrote:
> >
> > After this patch we can easily add another feature, pidfd_poll()
> > can add, say, POLLHUP to poll_flags if the pid is "dead".
> >
> > So the user can do
> >
> > 	poll(pidfd, { .revents = POLLHUP });
> >
> > and it will block until release_task() is called and this pid is
> > no longer in use (pid_task() == NULL).
> >
> > Do you think this can be useful?
>
> Yeah, I think this is something that people would find useful. IIUC, it
> would essentially allow them to do things like wait until a task has
> been waited upon

Exactly.

OK. I'll try to make the (hopefully simple) patch on top of this one
on Friday, if Tycho agrees with V3. Will be busy tomorrow.

> * systemd completely relying on pidfds to manage services to guard
>   against any pid races.
> * Extended dbus to allow authentication via pidfds.
> * Extended policy kit to enable secure authentication of processes via pidfds.
> * Language support for pidfds: Go, Rust etc.
> * An endless number of tools that added support for them.
> * glibc support for pidfd apis.
>
> There's a bunch more. That literally obliterated whole bug classes.

Thanks for this info!

Not that I ever thouhgt that pidfd is "useless", not at all, but as I said
(and as a Perl progammer ;) I simply do not know what people actually do with
pidfds ;)

Oleg.