Re: while_each_thread() under rcu_read_lock() is broken?

[Oleg Nesterov <oleg@redhat.com>]

On 06/21, Roland McGrath wrote:
>
> > Paul, Roland, do you see any problems from the correctness pov,
> > or a better fix for now?
> >
> > Perhaps it also makes sense to keep the old variant renamed to
> > while_each_thread_locked(), I dunno.
>
> Did we verify that only de_thread() can create the situation where a
> while_each_thread-style loop without either lock can be confused?

I think yes, this is is the only case.

I mean, while_each_thread(group_leader, t). If g != group_leader, then
the lockless while_each_thread() has problems with the plain exit(g).

Afaics. The more I think about this, the more I feel confused ;)

But if we start from ->group_leader, then while_each_thread() must
stop eventually. Otherwise we should assume that the dead (unhashed)
tasks can create the circular list, obviously this is not possible.

> If
> that's so, then just changing it to avoid the situation seems like it
> would be less invasive overall.

How? We should change ->group_leader uner write_lock_irq(tasklist),
synchronize_rcu() is not an option. We can't do call_rcu(release_task),
we can't take tasklist for writing in the softirq context. But even
if we could, this can't help in fact or I missed something.

Oleg.