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Date: Wed, 21 Dec 2011 19:11:01 +0100
From: Oleg Nesterov <oleg@redhat.com>
To: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Li Zefan <lizf@cn.fujitsu.com>, Tejun Heo <tj@kernel.org>,
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        KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>,
        Paul Menage <paul@paulmenage.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Subject: Re: Q: cgroup: Questions about possible issues in cgroup locking
Message-ID: <20111221181101.GA3092@redhat.com>
References: <20111221034334.GD17668@somewhere> <20111221130848.GA19679@redhat.com> <20111221175943.GG17668@somewhere>
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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.