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

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Date:   Thu, 18 Apr 2019 19:26:44 +0200
From:   Christian Brauner <christian@...uner.io>
To:     Jann Horn <jannh@...gle.com>, Oleg Nesterov <oleg@...hat.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        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 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.