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Date:   Thu, 18 Apr 2019 19:26:44 +0200
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Subject: Re: [PATCH RFC 1/2] Add polling support to pidfd
To:     Jann Horn <jannh@google.com>, Oleg Nesterov <oleg@redhat.com>,
        Joel Fernandes <joel@joelfernandes.org>,
        Florian Weimer <fweimer@redhat.com>
CC:     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>
From:   Christian Brauner <christian@brauner.io>
Message-ID: <CEB62F51-E8BF-4EFB-B438-8AF76633FF00@brauner.io>
Sender: linux-kernel-owner@vger.kernel.org
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On April 18, 2019 7:23:38 PM GMT+02:00, Jann Horn <jannh@google=2Ecom> wrot=
e:
>On Wed, Apr 17, 2019 at 3:09 PM Oleg Nesterov <oleg@redhat=2Ecom> 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=2E
>>
>> IOW, when the whole thread group exits, so it can't be used to
>monitor sub-threads=2E
>>
>> just in case=2E=2E=2E 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=2E /proc/sub-thread-tid has
>proc_tgid_base_operations despite not being a thread group leader=2E
>(Yes, that's kinda weird=2E) AFAICS the WARN_ON_ONCE() in this code can
>be hit trivially, and then the code will misbehave=2E
>
>@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=2E

The latter case probably being preferred if this API is supposed to be use=
able for thread management in userspace=2E