Re: [PATCH v2 2/6] sched: split out css_online/css_offline from tg creation/destruction

Ingo Molnar
2013-01-24T10:01:43Z

* Li Zefan <lizefan@huawei.com> wrote:

> 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>

Acked-by: Ingo Molnar <mingo@kernel.org>

Thanks,

Ingo