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Date: Wed, 4 Feb 2009 01:12:43 +0100
From: Oleg Nesterov <oleg@redhat.com>
To: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Ingo Molnar <mingo@elte.hu>, Steven Rostedt <srostedt@redhat.com>,
       linux-kernel@vger.kernel.org
Subject: Re: [PATCH] ftrace: do_each_pid_task() needs rcu lock
Message-ID: <20090204001243.GA6567@redhat.com>
References: <20090203193904.GA23695@redhat.com> <20090203194214.GA23703@redhat.com> <m1fxivw673.fsf@fess.ebiederm.org> <20090203222803.GA31756@redhat.com> <m1zlh3umho.fsf@fess.ebiederm.org>
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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.