Re: Q: cgroup: Questions about possible issues in cgroup locking

[Oleg Nesterov]

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.

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