cgroup: remove css_parent()

cgroup in general is moving towards using cgroup_subsys_state as the
fundamental structural component and css_parent() was introduced to
convert from using cgroup->parent to css->parent.  It was quite some
time ago and we're moving forward with making css more prominent.

This patch drops the trivial wrapper css_parent() and let the users
dereference css->parent.  While at it, explicitly mark fields of css
which are public and immutable.

v2: New usage from device_cgroup.c converted.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Michal Hocko <mhocko@suse.cz>
Acked-by: Neil Horman <nhorman@tuxdriver.com>
Acked-by: "David S. Miller" <davem@davemloft.net>
Acked-by: Li Zefan <lizefan@huawei.com>
Cc: Vivek Goyal <vgoyal@redhat.com>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Johannes Weiner <hannes@cmpxchg.org>

1 files changed, 11 insertions, 18 deletions

diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index 1737db0c63fe..2549493d518d 100644
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -48,22 +48,28 @@ enum cgroup_subsys_id {
 };
 #undef SUBSYS
 
-/* Per-subsystem/per-cgroup state maintained by the system. */
+/*
+ * Per-subsystem/per-cgroup state maintained by the system.  This is the
+ * fundamental structural building block that controllers deal with.
+ *
+ * Fields marked with "PI:" are public and immutable and may be accessed
+ * directly without synchronization.
+ */
 struct cgroup_subsys_state {
-        /* the cgroup that this css is attached to */
+        /* PI: the cgroup that this css is attached to */
         struct cgroup *cgroup;
 
-        /* the cgroup subsystem that this css is attached to */
+        /* PI: the cgroup subsystem that this css is attached to */
         struct cgroup_subsys *ss;
 
         /* reference count - access via css_[try]get() and css_put() */
         struct percpu_ref refcnt;
 
-        /* the parent css */
+        /* PI: the parent css */
         struct cgroup_subsys_state *parent;
 
         /*
-         * Subsys-unique ID.  0 is unused and root is always 1.  The
+         * PI: Subsys-unique ID.  0 is unused and root is always 1.  The
          * matching css can be looked up using css_from_id().
          */
         int id;
@@ -670,19 +676,6 @@ struct cgroup_subsys {
 #undef SUBSYS
 
 /**
- * css_parent - find the parent css
- * @css: the target cgroup_subsys_state
- *
- * Return the parent css of @css.  This function is guaranteed to return
- * non-NULL parent as long as @css isn't the root.
- */
-static inline
-struct cgroup_subsys_state *css_parent(struct cgroup_subsys_state *css)
-{
-        return css->parent;
-}
-
-/**
  * task_css_set_check - obtain a task's css_set with extra access conditions
  * @task: the task to obtain css_set for
  * @__c: extra condition expression to be passed to rcu_dereference_check()