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

[Christian Brauner]

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@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

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?

Christian

> - POLLIN  -> information can be read

> 
> Christian