From mboxrd@z Thu Jan  1 00:00:00 1970
From: Oleg Nesterov <oleg-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
Subject: Re: Q: cgroup: Questions about possible issues in cgroup locking
Date: Wed, 21 Dec 2011 19:11:01 +0100
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To: Frederic Weisbecker <fweisbec-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
Cc: Containers <containers-cunTk1MwBs9QetFLy7KEm3xJsTq8ys+cHZ5vskTnxNA@public.gmane.org>, LKML <linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org>, Paul Menage <paul-inf54ven1CmVyaH7bEyXVA@public.gmane.org>, Tejun Heo <tj-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>, Cgroups <cgroups-u79uwXL29TY76Z2rM5mHXA@public.gmane.org>, Andrew Morton <akpm-de/tnXTf+JLsfHDXvbKv3WD2FQJk+8+b@public.gmane.org>, "Paul E. McKenney" <paulmck-23VcF4HTsmIX0ybBhKVfKdBPR1lH4CV8@public.gmane.org>, Mandeep Singh Baines <msb-F7+t8E8rja9g9hUCZPvPmw@public.gmane.org>

On 12/21, Frederic Weisbecker wrote:
>
> On Wed, Dec 21, 2011 at 02:08:48PM +0100, Oleg Nesterov wrote:
> > On 12/21, Frederic Weisbecker wrote:
> > > - By the time we call cgroup_post_fork(), it is ready to be woken up
> > > and usable by the scheduler.
> >
> > No, the new child can't run until do_fork()->wake_up_new_task().
>
> Out of curiosity, why is it not possible for a task to kill and wake up the child
> before that happens?

Because it is not possible to wake it up.

Please note that copy_process() creates the "deactivated" child, iow
it is not on rq.

But, at the same time its ->state == TASK_RUNNING. This "fools"
try_to_wake_up() or anything else which in theory could place it
on the runqueue.

Except, of course, wake_up_new_task() does activate_task(). And
note that it does this unconditionally, exactly because we know that
this task can't be woken.

Oleg.