Re: [PATCH 2/3] pidfd_open.2: add PIDFD_THREAD and poll nuances

[Oleg Nesterov <oleg@redhat.com>]

Hi Kir,

On 07/08, Kir Kolyshkin wrote:
>
> [1]: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=64bef697d33b

The changelog says:

    pidfd: implement PIDFD_THREAD flag for pidfd_open()

    With this flag:

            - pidfd_open() doesn't require that the target task must be
              a thread-group leader

            - pidfd_poll() succeeds when the task exits and becomes a
              zombie (iow, passes exit_notify()), even if it is a leader
              and thread-group is not empty.

              This means that the behaviour of pidfd_poll(PIDFD_THREAD,
              pid-of-group-leader) is not well defined if it races with
              exec() from its sub-thread; pidfd_poll() can succeed or not
              depending on whether pidfd_task_exited() is called before
              or after exchange_tids().

> +The behavior depends on whether the file descriptor refers
> +to a process (thread-group leader) or a thread (see
> +.B PIDFD_THREAD
> +above):
> +.RS
> +.IP \[bu] 3
> +For a thread-group leader, the polling task is woken if the
> +thread-group is empty. In other words, if the thread-group
> +leader task exits when there are still threads alive in its
> +thread-group, the polling task will not be woken when the
> +thread-group leader exits, but rather when the last thread in the
> +thread-group exits.

so this part is not accurate.

See also 43f0df54c96fa5a ("pidfd_poll: report POLLHUP when pid_task() == NULL")
which adds another feature.

Oleg.