linux-kernel - Re: [GIT PULL] pidfd fixes

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Date:   Mon, 22 Jul 2019 18:39:34 +0200
From:   Christian Brauner <christian@...uner.io>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        Oleg Nesterov <oleg@...hat.com>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Joel Fernandes <joel@...lfernandes.org>
Subject: Re: [GIT PULL] pidfd fixes

On Mon, Jul 22, 2019 at 09:27:08AM -0700, Linus Torvalds wrote:
> On Mon, Jul 22, 2019 at 7:26 AM Christian Brauner <christian@...uner.io> wrote:
> >
> > This contains a fix for pidfd polling. It ensures that the task's exit
> > state is visible to all waiters:
> 
> Hmm.
> 
> I've pulled this, but the exit_state thing has been very fragile
> before, and I'm not entirely happy with how this just changes where it
> is set. I guess the movement here is all inside the tasklist_lock, so
> it's not that big of a deal, but still..
> 
> I would *really* like Oleg to take a look.

Oh, sorry. Oleg did take a look. See:

https://lore.kernel.org/lkml/20190718150057.GB13355@redhat.com/
https://lore.kernel.org/lkml/20190719161404.GA24170@redhat.com/

> 
> Also, and the primary reason I write this email is that this basically
> makes the "EXIT_ZOMBIE / EXIT_DEAD" state handling look all kinds of
> crazy. You set it to EXIT_ZOMBIE potentially _twice_. Whaa?
> 
> So if we set EXIT_ZOMBIE early, then I think we should change the
> EXIT_DEAD case too. IOW, do something like this on top:
> 
>   --- a/kernel/exit.c
>   +++ b/kernel/exit.c
>   @@ -734,9 +734,10 @@ static void exit_notify(struct task_struct
> *tsk, int group_dead)
>                 autoreap = true;
>         }
> 
>   -     tsk->exit_state = autoreap ? EXIT_DEAD : EXIT_ZOMBIE;
>   -     if (tsk->exit_state == EXIT_DEAD)
>   +     if (autoreap) {
>   +             tsk->exit_state = EXIT_DEAD;
>                 list_add(&tsk->ptrace_entry, &dead);
>   +     }
> 
>         /* mt-exec, de_thread() is waiting for group leader */
>         if (unlikely(tsk->signal->notify_count < 0))
> 
> where now the logic becomes "ok, we turned into a zombie above, and if
> we autoreap this thread then we turn the zombie into a fully dead
> thread".
> 
> Because currently we end up having "first turn it into a zombie", then
> "set it to zombie or dead depending on autoreap" and then "if we
> turned it into dead, move it to the dead list".
> 
> That just feels confused to me. Comments?

Agreed. But that codepath is so core-kernel that I really felt more
comfortable just doing the absolut minimal thing so that when things
bite us we see it right away.

There's no harm in sending a cleanup for this later I think, when we
haven't hit any weirdness with the current change. Does that sound ok?

Christian