cgroup_freezer: add ->post_create() and ->pre_destroy() and track online state
A cgroup is online and visible to iteration between ->post_create() and ->pre_destroy(). This patch introduces CGROUP_FREEZER_ONLINE and toggles it from the newly added freezer_post_create() and freezer_pre_destroy() while holding freezer->lock such that a cgroup_freezer can be reilably distinguished to be online. This will be used by full hierarchy support. ONLINE test is added to freezer_apply_state() but it currently doesn't make any difference as freezer_write() can only be called for an online cgroup. Adjusting system_freezing_cnt on destruction is moved from freezer_destroy() to the new freezer_pre_destroy() for consistency. This patch doesn't introduce any noticeable behavior change. Signed-off-by: Tejun Heo <tj@kernel.org> Reviewed-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> Reviewed-by: Michal Hocko <mhocko@suse.cz>
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