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

[Oleg Nesterov]

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.