Re: [PATCH RFC 1/2] Add polling support to pidfd

[Christian Brauner <christian@brauner.io>]

On Fri, Apr 19, 2019 at 09:22:33PM +0200, Christian Brauner wrote:
> On Fri, Apr 19, 2019 at 09:18:58PM +0200, Christian Brauner wrote:
> > On Fri, Apr 19, 2019 at 03:02:47PM -0400, Joel Fernandes wrote:
> > > On Thu, Apr 18, 2019 at 07:26:44PM +0200, Christian Brauner wrote:
> > > > On April 18, 2019 7:23:38 PM GMT+02:00, Jann Horn <jannh@google.com> wrote:
> > > > >On Wed, Apr 17, 2019 at 3:09 PM Oleg Nesterov <oleg@redhat.com> wrote:
> > > > >> On 04/16, Joel Fernandes wrote:
> > > > >> > On Tue, Apr 16, 2019 at 02:04:31PM +0200, Oleg Nesterov wrote:
> > > > >> > >
> > > > >> > > Could you explain when it should return POLLIN? When the whole
> > > > >process exits?
> > > > >> >
> > > > >> > It returns POLLIN when the task is dead or doesn't exist anymore,
> > > > >or when it
> > > > >> > is in a zombie state and there's no other thread in the thread
> > > > >group.
> > > > >>
> > > > >> IOW, when the whole thread group exits, so it can't be used to
> > > > >monitor sub-threads.
> > > > >>
> > > > >> just in case... speaking of this patch it doesn't modify
> > > > >proc_tid_base_operations,
> > > > >> so you can't poll("/proc/sub-thread-tid") anyway, but iiuc you are
> > > > >going to use
> > > > >> the anonymous file returned by CLONE_PIDFD ?
> > > > >
> > > > >I don't think procfs works that way. /proc/sub-thread-tid has
> > > > >proc_tgid_base_operations despite not being a thread group leader.
> > > > >(Yes, that's kinda weird.) AFAICS the WARN_ON_ONCE() in this code can
> > > > >be hit trivially, and then the code will misbehave.
> > > > >
> > > > >@Joel: I think you'll have to either rewrite this to explicitly bail
> > > > >out if you're dealing with a thread group leader, or make the code
> > > > >work for threads, too.
> > > > 
> > > > The latter case probably being preferred if this API is supposed to be
> > > > useable for thread management in userspace.
> > > 
> > > At the moment, we are not planning to use this for sub-thread management. I
> > > am reworking this patch to only work on clone(2) pidfds which makes the above
> > 
> > Indeed and agreed.
> > 
> > > discussion about /proc a bit unnecessary I think. Per the latest CLONE_PIDFD
> > > patches, CLONE_THREAD with pidfd is not supported.
> > 
> > Yes. We have no one asking for it right now and we can easily add this
> > later.
> > 
> > Admittedly I haven't gotten around to reviewing the patches here yet
> > completely. But one thing about using POLLIN. FreeBSD is using POLLHUP
> > on process exit which I think is nice as well. How about returning
> > POLLIN | POLLHUP on process exit?
> > We already do things like this. For example, when you proxy between
> > ttys. If the process that you're reading data from has exited and closed
> > it's end you still can't usually simply exit because it might have still
> > buffered data that you want to read.  The way one can deal with this
> > from  userspace is that you can observe a (POLLHUP | POLLIN) event and
> > you keep on reading until you only observe a POLLHUP without a POLLIN
> > event at which point you know you have read
> > all data.
> > I like the semantics for pidfds as well as it would indicate:
> > - POLLHUP -> process has exited
> 
> or POLLRDHUP. The check you'd usually perform would probably be
> if ((revents & (POLLIN | POLLPRI)) > 0) && ((revents & (POLLHUP | POLLRDHUP)) > 0)
>         /* keep on trying to read */
> 
> I guess you have that set of flags already suggested in another mail?

The code where this pattern is e.g. used is in drivers/tty/n_tty.c:

static __poll_t n_tty_poll(struct tty_struct *tty, struct file *file,
                                                        poll_table *wait)
{
        __poll_t mask = 0;
 
        poll_wait(file, &tty->read_wait, wait);
        poll_wait(file, &tty->write_wait, wait);
        if (input_available_p(tty, 1))
                mask |= EPOLLIN | EPOLLRDNORM;
        else {
                tty_buffer_flush_work(tty->port);
                if (input_available_p(tty, 1))
                        mask |= EPOLLIN | EPOLLRDNORM;
        }
        if (tty->packet && tty->link->ctrl_status)
                mask |= EPOLLPRI | EPOLLIN | EPOLLRDNORM;
        if (test_bit(TTY_OTHER_CLOSED, &tty->flags))
                mask |= EPOLLHUP;
        if (tty_hung_up_p(file))
                mask |= EPOLLHUP;
        if (tty->ops->write && !tty_is_writelocked(tty) &&
                        tty_chars_in_buffer(tty) < WAKEUP_CHARS &&
                        tty_write_room(tty) > 0)
                mask |= EPOLLOUT | EPOLLWRNORM;
        return mask;
}