cgroup: always use cgroup_next_child() to walk the children list

There are several places where the children list is accessed directly.
This patch converts those places to use cgroup_next_child().  This
will help updating the hierarchy iterators to use @css instead of
@cgrp.

While cgroup_next_child() can be heavy in pathological cases - e.g. a
lot of dead children, this shouldn't cause any noticeable behavior
differences.

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

2 files changed, 5 insertions, 7 deletions

diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index 5f9ba5881717..c288bce428f8 100644
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -800,9 +800,8 @@ struct cgroup *cgroup_next_child(struct cgroup *pos, struct cgroup *cgrp);
  * the start of the next iteration by, for example, bumping the css refcnt.
  */
 #define cgroup_for_each_child(pos, cgrp)                                \
-        for ((pos) = list_first_or_null_rcu(&(cgrp)->children,          \
-                                            struct cgroup, sibling);    \
-             (pos); (pos) = cgroup_next_child((pos), (cgrp)))
+        for ((pos) = cgroup_next_child(NULL, (cgrp)); (pos);            \
+             (pos) = cgroup_next_child((pos), (cgrp)))
 
 struct cgroup *cgroup_next_descendant_pre(struct cgroup *pos,
                                           struct cgroup *cgroup);
diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index dd55244952bd..2b7354faaca7 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -3112,7 +3112,7 @@ struct cgroup *cgroup_next_descendant_pre(struct cgroup *pos,
                 pos = cgroup;
 
         /* visit the first child if exists */
-        next = list_first_or_null_rcu(&pos->children, struct cgroup, sibling);
+        next = cgroup_next_child(NULL, pos);
         if (next)
                 return next;
 
@@ -3151,7 +3151,7 @@ struct cgroup *cgroup_rightmost_descendant(struct cgroup *pos)
                 last = pos;
                 /* ->prev isn't RCU safe, walk ->next till the end */
                 pos = NULL;
-                list_for_each_entry_rcu(tmp, &last->children, sibling)
+                cgroup_for_each_child(tmp, last)
                         pos = tmp;
         } while (pos);
 
@@ -3165,8 +3165,7 @@ static struct cgroup *cgroup_leftmost_descendant(struct cgroup *pos)
 
         do {
                 last = pos;
-                pos = list_first_or_null_rcu(&pos->children, struct cgroup,
-                                             sibling);
+                pos = cgroup_next_child(NULL, pos);
         } while (pos);
 
         return last;