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

[Christian Brauner]

On Fri, Apr 19, 2019 at 11:21 PM Daniel Colascione <dancol@xxxxxxxxxx> wrote:
>
> On Fri, Apr 19, 2019 at 1:57 PM Christian Brauner <christian@xxxxxxxxxx> wrote:
> >
> > On Fri, Apr 19, 2019 at 10:34 PM Daniel Colascione <dancol@xxxxxxxxxx> wrote:
> > >
> > > On Fri, Apr 19, 2019 at 12:49 PM Joel Fernandes <joel@xxxxxxxxxxxxxxxxx> 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@xxxxxxxxxx> wrote:
> > > > > > > >On Wed, Apr 17, 2019 at 3:09 PM Oleg Nesterov <oleg@xxxxxxxxxx> 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.
>
> Sure, but systemd is specialized.

So is Android and we're not designing an interface for Android but for
all of userspace.
I hope this is clear. Service managers are quite important and systemd
is the largest one
and they can make good use of this feature.

>
> There are two broad classes of programs that care about process exit
> status: 1) those that just want to do something and wait for it to
> complete, and 2) programs that want to perform detailed monitoring of
> processes and intervention in their state. #1 is overwhelmingly more
> common. The basic pidfd feature should take care of case #1 only, as
> wait*() in file descriptor form. I definitely don't think we should be
> complicating the interface and making it more error-prone (see below)
> for the sake of that rare program that cares about non-exit
> notification conditions. You're proposing a complicated combination of
> poll bit flags that most users (the ones who just wait to wait for
> processes) don't care about and that risk making the facility hard to
> use with existing event loops, which generally recognize readability
> and writability as the only properties that are worth monitoring.

That whole pargraph is about dismissing a range of valid use-cases based on
assumptions such as "way more common" and
even argues that service managers are special cases and therefore not
really worth considering. I would like to be more open to other use cases.

>
> > 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.
>
> Exit status information should be *at least* as broadly available
> through pidfds as it is through the last field of /proc/pid/stat
> today, and probably more broadly. I've been saying for six months now
> that we need to talk about *who* should have access to exit status
> information. We haven't had that conversation yet. My preference is to

> just make exit status information globally available, as FreeBSD seems
> to do. I think it would be broadly useful for something like pkill to

>From the pdfork() FreeBSD manpage:
"poll(2) and select(2) allow waiting for process state transitions;
currently only POLLHUP is defined, and will be raised when the process dies.
Process state transitions can also be monitored using kqueue(2) filter
EVFILT_PROCDESC; currently only NOTE_EXIT is implemented."

> wait for processes to exit and to retrieve their exit information.
>
> Speaking of pkill: AIUI, in your current patch set, one can get a
> pidfd *only* via clone. Joel indicated that he believes poll(2)
> shouldn't be supported on procfs pidfds. Is that your thinking as
> well? If that's the case, then we're in a state where non-parents

Yes, it is.

> can't wait for process exit, and providing this facility is an
> important goal of the whole project.

That's your goal.

>
> > 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().
>
> And do you imagine making read() destructive? Does that read() then
> reset the POLLIN state? You're essentially proposing that a pidfd
> provide an "event stream" interface, delivering notifications packets
> that indicate state changes like "process exited" or "process stopped"
> or "process execed". While this sort of interface is powerful and has
> some nice properties that tools like debuggers and daemon monitors
> might want to use, I think it's too complicated and error prone for

That's an assumption again.

> the overwhelmingly common case of wanting to monitor process lifetime.

Again where is this assumption backed up? systemd is a valid example where
they care about this, various container managers are another one, and FreeBSD
already does this.

> I'd much rather pidfd provide a simple one-state-transition
> level-triggered (not edge-triggered, as your suggestion implies)
> facility. If we want to let sophisticated programs read a stream of
> notification packets indicating changes in process state, we can
> provide that as a separate interface in future work.
>
> I like Linus' idea of just making waitid(2) (not waitpid(2), as I
> mistakenly mentioned earlier) on a pidfd act *exactly* like a
> waitid(2) on the corresponding process and making POLLIN just mean
> "waitid will succeed". It's a nice simple model that's easy to reason
> about and that makes it easy to port existing code to pidfds.
>
> I am very much against signaling additional information on basic
> pidfds using non-POLLIN poll flags.