Re: [PATCH v3 0/1] pidfd: implement PIDFD_THREAD flag for pidfd_open()

[Christian Brauner <brauner@kernel.org>]

On Wed, Jan 31, 2024 at 03:12:04PM +0100, Oleg Nesterov wrote:
> Before I forget this...
> 
> 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 which for service managers is very useful information.

---

Btw, bigger picture for a second. You probably haven't kept track of
this but the fact that we got pidfds - thanks a lot to your review and
help - has made quite a huge difference in userspace. Since we last did
any meaningful work we got:

* 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.

I think with PIDFD_THREAD support it might make it interesting to use it
for some pthread management as well.