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

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Date:   Wed, 17 Apr 2019 15:09:41 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     Joel Fernandes <joel@...lfernandes.org>
Cc:     linux-kernel@...r.kernel.org, luto@...capital.net,
        rostedt@...dmis.org, dancol@...gle.com, christian@...uner.io,
        jannh@...gle.com, surenb@...gle.com, 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@...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@...roid.com,
        Tycho Andersen <tycho@...ho.ws>
Subject: Re: [PATCH RFC 1/2] Add polling support to pidfd

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 ?


> > Then all you need is
> >
> > 	!task || task->exit_state && thread_group_empty(task)
>
> Yes this works as well, all the tests pass with your suggestion so I'll
> change it to that. Although I will the be giving up returing EPOLLERR if the
> task_struct doesn't exit. We don't need that, but I thought it was cool to
> return it anyway.

OK, task == NULL means that it was already reaped by parent, pid_nr is free,
probably useful....

> > Please do not use EXIT_DEAD/EXIT_ZOMBIE. And ->wait_pidfd should probably
> > live in task->signal_struct.
>
> About wait_pidfd living in signal_struct, that wont work since the waitqueue
> has to survive for the duration of the poll system call.

That is why I said this will need the additional cleanup in free_signal_struct().
But I was wrong, somehow I forgot that free_poll_entry() needs wq_head->lock ;)
so this will need much more complications, lets forget it...

> Also the waitqueue living in struct pid solves the de_thread() issue I
> mentioned later in the following thread and in the commit message:
> https://lore.kernel.org/patchwork/comment/1257175/

Hmm...

	2. By including the struct pid for the waitqueue means that during
	de_exec, the thread doing de_thread() automatically gets the new
	waitqueue/pid even though its task_struct is different.

this one?

this is not true, or I do not understand...

it gets the _same_ (old, not new) PIDTYPE_TGID pid even if it changes task_struct.
But probably this is what you actually meant, because this is what your patch wants
or I am totally confused.

And note that exec/de_thread doesn't change ->signal_struct, so I do not understand
you anyway. Nevermind.

Oleg.