cgroup: reorganize css_task_iter

This patch reorganizes css_task_iter so that adding effective css
support is easier.

* s/->cset_link/->cset_pos/ and s/->task/->task_pos/ for consistency

* ->origin_css is used to determine whether the iteration reached the
  last css_set.  Replace it with explicit ->cset_head so that
  css_advance_task_iter() doesn't have to know the termination
  condition directly.

* css_task_iter_next() currently assumes that it's walking list of
  cgrp_cset_link and reaches into the current cset through the current
  link to determine the termination conditions for task walking.  As
  this won't always be true for effective css walking, add
  ->tasks_head and ->mg_tasks_head and use them to control task
  walking so that css_task_iter_next() doesn't have to know how
  css_sets are being walked.

This patch doesn't make any behavior changes.  The iteration logic
stays unchanged after the patch.

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

1 files changed, 6 insertions, 3 deletions

diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index 33a0043ef454..bee390586120 100644
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -842,9 +842,12 @@ css_next_descendant_post(struct cgroup_subsys_state *pos,
 
 /* A css_task_iter should be treated as an opaque object */
 struct css_task_iter {
-	struct cgroup_subsys_state	*origin_css;
-	struct list_head		*cset_link;
-	struct list_head		*task;
+	struct list_head		*cset_pos;
+	struct list_head		*cset_head;
+
+	struct list_head		*task_pos;
+	struct list_head		*tasks_head;
+	struct list_head		*mg_tasks_head;
 };
 
 void css_task_iter_start(struct cgroup_subsys_state *css,