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

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Date:   Fri, 19 Apr 2019 15:02:47 -0400
From:   Joel Fernandes <joel@...lfernandes.org>
To:     Christian Brauner <christian@...uner.io>
Cc:     Jann Horn <jannh@...gle.com>, Oleg Nesterov <oleg@...hat.com>,
        Florian Weimer <fweimer@...hat.com>,
        kernel list <linux-kernel@...r.kernel.org>,
        Andy Lutomirski <luto@...capital.net>,
        Steven Rostedt <rostedt@...dmis.org>,
        Daniel Colascione <dancol@...gle.com>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Alexey Dobriyan <adobriyan@...il.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Andrei Vagin <avagin@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Arnd Bergmann <arnd@...db.de>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Kees Cook <keescook@...omium.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        linux-kselftest@...r.kernel.org, Michal Hocko <mhocko@...e.com>,
        Nadav Amit <namit@...are.com>, Serge Hallyn <serge@...lyn.com>,
        Shuah Khan <shuah@...nel.org>,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        Taehee Yoo <ap420073@...il.com>, Tejun Heo <tj@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        kernel-team <kernel-team@...roid.com>,
        Tycho Andersen <tycho@...ho.ws>
Subject: Re: [PATCH RFC 1/2] Add polling support to pidfd

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@...gle.com> wrote:
> >On Wed, Apr 17, 2019 at 3:09 PM Oleg Nesterov <oleg@...hat.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
discussion about /proc a bit unnecessary I think. Per the latest CLONE_PIDFD
patches, CLONE_THREAD with pidfd is not supported.

Also we wanted to make the polling of pidfd quite close to the wait(2) family
semantics which, as I understand correctly, is not something that works for
sub-threads. In the future, we could bail in poll(2) or return an error, if
clone(2) starts supporting thread pidfds, but at the moment I will like to
keep the WARN_ON just in case. Please let me know if I missed something.

thanks!

 - Joel