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Date:   Fri, 19 Apr 2019 21:22:34 +0200
From:   Christian Brauner <christian@brauner.io>
To:     Joel Fernandes <joel@joelfernandes.org>
Cc:     Jann Horn <jannh@google.com>, Oleg Nesterov <oleg@redhat.com>,
        Florian Weimer <fweimer@redhat.com>,
        kernel list <linux-kernel@vger.kernel.org>,
        Andy Lutomirski <luto@amacapital.net>,
        Steven Rostedt <rostedt@goodmis.org>,
        Daniel Colascione <dancol@google.com>,
        Suren Baghdasaryan <surenb@google.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Alexey Dobriyan <adobriyan@gmail.com>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Andrei Vagin <avagin@gmail.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Arnd Bergmann <arnd@arndb.de>,
        "Eric W. Biederman" <ebiederm@xmission.com>,
        Kees Cook <keescook@chromium.org>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        linux-kselftest@vger.kernel.org, Michal Hocko <mhocko@suse.com>,
        Nadav Amit <namit@vmware.com>, Serge Hallyn <serge@hallyn.com>,
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        Stephen Rothwell <sfr@canb.auug.org.au>,
        Taehee Yoo <ap420073@gmail.com>, Tejun Heo <tj@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        kernel-team <kernel-team@android.com>,
        Tycho Andersen <tycho@tycho.ws>
Subject: Re: [PATCH RFC 1/2] Add polling support to pidfd
Message-ID: <20190419192233.d2n6al7hgoga6gts@brauner.io>
References: <20190411175043.31207-1-joel@joelfernandes.org>
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 <20190416192051.GA184889@google.com>
 <20190417130940.GC32622@redhat.com>
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 <CEB62F51-E8BF-4EFB-B438-8AF76633FF00@brauner.io>
 <20190419190247.GB251571@google.com>
 <20190419191858.iwcvqm6fihbkaata@brauner.io>
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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@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

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