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

[Daniel Colascione]

On Fri, Apr 19, 2019 at 2:48 PM Christian Brauner <christian@xxxxxxxxxx> wrote:
>
> 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.

Nothing in my post is Android-specific. Waiting for non-child
processes is something that lots of people want to do, which is why
patches to enable it have been getting posted every few years for many
years (e.g., Andy's from 2011). I, too, want to make an API for all
over userspace. Don't attribute to me arguments that I'm not actually
making.

> 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.

Service managers already have the tools they need to do their job. The
kind of monitoring you're talking about is a niche case and an
improved API for this niche --- which amounts to a rethought ptrace
--- can wait for a future date, when it can be done right. Nothing in
the model I'm advocating precludes adding an event stream API in the
future. I don't think we should gate the ability to wait for process
exit via pidfd on pidfds providing an entire ptrace replacement
facility.

> > 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

It really ought not to be controversial to say that process managers
make up a small fraction of the programs that wait for child
processes.

> 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.

It's not my position that service managers are "not worth considering"
and you know that, so I'd appreciate your not attributing to me views
hat I don't hold. I *am* saying that an event-based process-monitoring
API is out of scope and that it should be separate work: the
overwhelmingly majority of process manipulation (say, in libraries
wanting private helper processes, which is something I thought we all
agreed would be beneficial to support) is waiting for exit.

> > > 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."

I don't understand what you're trying to demonstrate by quoting that passage.

> > 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.

If reading process status information from a pidfd is destructive,
it's dangerous to share pidfds between processes. If reading
information *isn't* destructive, how are you supposed to use poll(2)
to wait for the next transition? Is poll destructive? If you can only
make a new pidfd via clone, you can't get two separate event streams
for two different users. Sharing a single pidfd via dup or SCM_RIGHTS
becomes dangerous, because if reading status is destructive, only one
reader can observe each event. Your proposed edge-triggered design
makes pidfds significantly less useful, because in your design, it's
unsafe to share a single pidfd open file description *and* there's no
way to create a new pidfd open file description for an existing
process.

I think we should make an API for all of userspace and not just for
container managers and systemd.

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

I thought we all agreed on that months ago that it's reasonable to
allow processes to wait for non-child processes to exit. Now, out of
the blue, you're saying that 1) actually, we want a rich API for all
kinds of things that aren't process exit, because systemd, and 2)
actually, non-parents shouldn't be able to wait for process death. I
don't know what to say. Can you point to something that might have
changed your mind? I'd appreciate other people weighing in on this
subject.