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

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Message-ID: <CAG48ez2Q0t2-rZz8wmGeih=NigYL-CT3+E6NgiPT0AkOQAUPMQ@mail.gmail.com>
Date:   Thu, 18 Apr 2019 19:23:38 +0200
From:   Jann Horn <jannh@...gle.com>
To:     Oleg Nesterov <oleg@...hat.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Christian Brauner <christian@...uner.io>,
        Florian Weimer <fweimer@...hat.com>
Cc:     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 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.