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Date:   Fri, 19 Apr 2019 17:43:54 +0200
From:   Oleg Nesterov <oleg@redhat.com>
To:     Jann Horn <jannh@google.com>
Cc:     Joel Fernandes <joel@joelfernandes.org>,
        Christian Brauner <christian@brauner.io>,
        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>,
        Shuah Khan <shuah@kernel.org>,
        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: <20190419154354.GB12228@redhat.com>
References: <20190411175043.31207-1-joel@joelfernandes.org>
 <20190416120430.GA15437@redhat.com>
 <20190416192051.GA184889@google.com>
 <20190417130940.GC32622@redhat.com>
 <CAG48ez2Q0t2-rZz8wmGeih=NigYL-CT3+E6NgiPT0AkOQAUPMQ@mail.gmail.com>
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On 04/18, Jann Horn 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, sorry, I meant /proc/pid/task/sub-thread-tid.

But poll("/proc/sub-thread-tid") won't work too, we can't rely on do_notify_parent()
if the task is not a 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.

Heh, I didn't even notice that WARN_ON_ONCE(task && !thread_group_leader(task)) ;)

> @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 last version of CLONE_PIDFD doesn't allow CLONE_THREAD, so we can forget
about this problem for now.

Oleg.