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

[Christian Brauner <christian@brauner.io>]

On Fri, Apr 19, 2019 at 10:34 PM Daniel Colascione <dancol@google.com> wrote:
>
> On Fri, Apr 19, 2019 at 12:49 PM Joel Fernandes <joel@joelfernandes.org> wrote:
> >
> > On Fri, Apr 19, 2019 at 09:18:59PM +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
> > > - POLLIN  -> information can be read
> >
> > Actually I think a bit different about this, in my opinion the pidfd should
> > always be readable (we would store the exit status somewhere in the future
> > which would be readable, even after task_struct is dead). So I was thinking
> > we always return EPOLLIN.  If process has not exited, then it blocks.
>
> ITYM that a pidfd polls as readable *once a task exits* and stays
> readable forever. Before a task exit, a poll on a pidfd should *not*
> yield POLLIN and reading that pidfd should *not* complete immediately.
> There's no way that, having observed POLLIN on a pidfd, you should
> ever then *not* see POLLIN on that pidfd in the future --- it's a
> one-way transition from not-ready-to-get-exit-status to
> ready-to-get-exit-status.

What do you consider interesting state transitions? A listener on a pidfd
in epoll_wait() might be interested if the process execs for example.
That's a very valid use-case for e.g. systemd.
We can't use EPOLLIN for that too otherwise you'd need to to waitid(_WNOHANG)
to check whether an exit status can be read which is not nice and then you
multiplex different meanings on the same bit.
I would prefer if the exit status can only be read from the parent which is
clean and the least complicated semantics, i.e. Linus waitid() idea.
EPOLLIN on a pidfd could very well mean that data can be read via
a read() on the pidfd *other* than the exit status. The read could e.g.
give you a lean struct that indicates the type of state transition: NOTIFY_EXIT,
NOTIFY_EXEC, etc.. This way we are not bound to a specific poll event indicating
a specific state.
Though there's a case to be made that EPOLLHUP could indicate process exit
and EPOLLIN a state change + read().