Re: [PATCH] ftrace: do_each_pid_task() needs rcu lock

[Oleg Nesterov <oleg@redhat.com>]

On 02/03, Eric W. Biederman wrote:
>
> Oleg Nesterov <oleg@redhat.com> writes:
>
> > Yes, I thought about de_thread() too. But we can't "fix" do_each_pid_task()
> > to avoid the race?
>
> For the case of exec there is that.  There is also the case that
> ftrace unlike everything else wants to trace be able to trace all of
> the idle threads with pid 0.  I think that is a special case
> currently, but for that case the only correct version I can think
> of do_each_task_pid(), and current do_each_task_pid is wrong because
> it does not allow that.

Well, yes, I (partly) agree.

But, if we are talking about idle threads, we should change copy_process()
first. Because fork_idle()->copy_process(..., pid => init_struct_pid, ...)
means we don't really attach the idle thread to init_struct_pid.

I must admit, I think we should keep init_struct_pid "special" anyway,
but in any case you are right imho, there are nasty oddities here.

Oleg.