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Date: Wed, 31 Aug 2011 23:54:26 +0200
From: Frederic Weisbecker <fweisbec@gmail.com>
To: Paul Menage <paul@paulmenage.org>
Cc: Kay Sievers <kay.sievers@vrfy.org>, Li Zefan <lizf@cn.fujitsu.com>,
        Tim Hockin <thockin@hockin.org>,
        Andrew Morton <akpm@linux-foundation.org>,
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        Johannes Weiner <hannes@cmpxchg.org>,
        Aditya Kali <adityakali@google.com>, Oleg Nesterov <oleg@redhat.com>
Subject: Re: [RFD] Task counter: cgroup core feature or cgroup subsystem?
 (was Re: [PATCH 0/8 v3] cgroups: Task counter subsystem)
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On Fri, Aug 26, 2011 at 08:16:32AM -0700, Paul Menage wrote:
> On Wed, Aug 24, 2011 at 10:54 AM, Frederic Weisbecker
> <fweisbec@gmail.com> wrote:
> >
> > It seems your patch doesn't handle the ->fork() and ->exit() calls.
> > We probably need a quick access to states of multi-subsystems from
> > the task, some lists available from task->cgroups, I don't know yet.
> >
> 
> That state is available, but currently only while holding cgroup_mutex
> - at least, that's what task_cgroup_from_root() requires.
> 
> It might be the case that we could achieve the same effect by just
> locking the task, so the pre-condition for task_cgroup_from_root()
> would be either that cgroup_mutex is held or the task lock is held.

Now I realize, is it necessary if we only want to access a subsys state
through task->cgroups->subsys[i] from the ->fork() callback?

The task is not yet added to the thread_group, its pid is not yet
hashed so I guess it can not concurrently be moved to another
cgroup.

We need a special flavour of task->cgroups->subsys for multi bindable
subsystem but that's the same.