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

[ebiederm@xmission.com (Eric W. Biederman)]

Oleg Nesterov <oleg@redhat.com> writes:

> 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.

We already do: call_rcu(&p->rcu, delayed_put_task_struct); in release_task.
We don't call release_task until after we have removed it as leader and
dropped the write lock. 

At first glance it sounds like the group leader is safe as a stopping
point for a rcu while_each_thread, and I expect the fact that
de_thread takes everything down to a single thread, could have nice
properties here.  If pid_alive were only to fail on the group leader
when de_thread is called I think we could legitimately say that an event
we won't worry about.  It is close enough to a new thread being created
anyway.

Eric