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

From: Oleg Nesterov <oleg@redhat.com>
To: Joel Fernandes <joel@joelfernandes.org>
Cc: linux-kernel@vger.kernel.org, luto@amacapital.net,
	rostedt@goodmis.org, dancol@google.com, christian@brauner.io,
	jannh@google.com, surenb@google.com,
	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@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@android.com, Tycho Andersen <tycho@tycho.ws>
Subject: Re: [PATCH RFC 1/2] Add polling support to pidfd
Date: Wed, 17 Apr 2019 15:09:41 +0200	[thread overview]
Message-ID: <20190417130940.GC32622@redhat.com> (raw)
In-Reply-To: <20190416192051.GA184889@google.com>

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.