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

[Christian Brauner]

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

Christian