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SubjectRe: [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@brauner.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

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