cgroup: replace cgroup->css_kill_cnt with ->nr_css

Currently, css (cgroup_subsys_state) lifetime is tied to that of the
associated cgroup.  With the planned unified hierarchy, css's will be
dynamically created and destroyed within the lifetime of a cgroup.  To
enable such usages, css's will be individually RCU protected instead
of being tied to the cgroup.

cgroup->css_kill_cnt is used during cgroup destruction to wait for css
reference count disable; however, this model doesn't work once css's
lifetimes are managed separately from cgroup's.  This patch replaces
it with cgroup->nr_css which is an cgroup_mutex protected integer
counting the number of attached css's.  The count is incremented from
online_css() and decremented after refcnt kill is confirmed.  If the
count reaches zero and the cgroup is marked dead, the second stage of
cgroup destruction is kicked off.  If a cgroup doesn't have any css
attached at the time of rmdir, cgroup_destroy_locked() now invokes the
second stage directly as no css kill confirmation would happen.

cgroup_offline_fn() - the second step of cgroup destruction - is
renamed to cgroup_destroy_css_killed() and now expects to be called
with cgroup_mutex held.

While this patch changes how css destruction is punted to work items,
it shouldn't change any visible behavior.

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

1 files changed, 3 insertions, 1 deletions

diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index eb200b5794e7..80dca872f4d4 100644
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -171,6 +171,9 @@ struct cgroup {
          */
         int id;
 
+        /* the number of attached css's */
+        int nr_css;
+
         /*
          * We link our 'sibling' struct into our parent's 'children'.
          * Our children link their 'sibling' into our 'children'.
@@ -234,7 +237,6 @@ struct cgroup {
         /* For css percpu_ref killing and RCU-protected deletion */
         struct rcu_head rcu_head;
         struct work_struct destroy_work;
-        atomic_t css_kill_cnt;
 
         /* List of events which userspace want to receive */
         struct list_head event_list;