From: Li Zefan <lizefan@huawei.com>
Date: Thu, 24 Jan 2013 06:30:48 +0000 (+0800)
Subject: sched: split out css_online/css_offline from tg creation/destruction
X-Git-Tag: v3.9-rc1~157^2~8
X-Git-Url: http://git.openpandora.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=ace783b9bbfa2182b4a561498db3f09a0c56bc79;p=pandora-kernel.git

sched: split out css_online/css_offline from tg creation/destruction

This is a preparaton for later patches.

- What do we gain from cpu_cgroup_css_online():

After ss->css_alloc() and before ss->css_online(), there's a small
window that tg->css.cgroup is NULL. With this change, tg won't be seen
before ss->css_online(), where it's added to the global list, so we're
guaranteed we'll never see NULL tg->css.cgroup.

- What do we gain from cpu_cgroup_css_offline():

tg is freed via RCU, so is cgroup. Without this change, This is how
synchronization works:

cgroup_rmdir()
  no ss->css_offline()
diput()
  syncornize_rcu()
  ss->css_free()       <-- unregister tg, and free it via call_rcu()
  kfree_rcu(cgroup)    <-- wait possible refs to cgroup, and free cgroup

We can't just kfree(cgroup), because tg might access tg->css.cgroup.

With this change:

cgroup_rmdir()
  ss->css_offline()    <-- unregister tg
diput()
  synchronize_rcu()    <-- wait possible refs to tg and cgroup
  ss->css_free()       <-- free tg
  kfree_rcu(cgroup)    <-- free cgroup

As you see, kfree_rcu() is redundant now.

Signed-off-by: Li Zefan <lizefan@huawei.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Ingo Molnar <mingo@kernel.org>
---

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