cgroup: reorganize css_task_iter functions

* Rename css_advance_task_iter() to css_task_iter_advance_css_set()
  and make it clear it->task_pos too at the end of the iteration.

* Factor out css_task_iter_advance() from css_task_iter_next().  The
  new function whines if called on a terminated iterator.

Except for the termination check, this is pure reorganization and
doesn't introduce any behavior changes.  This will help the planned
locking update for css_task_iter.

Signed-off-by: Tejun Heo <tj@kernel.org>

1 files changed, 28 insertions, 21 deletions

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 0b6e50c67b65..56e2b772b143 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -3793,12 +3793,12 @@ bool css_has_online_children(struct cgroup_subsys_state *css)
 }
 
 /**
- * css_advance_task_iter - advance a task itererator to the next css_set
+ * css_task_iter_advance_css_set - advance a task itererator to the next css_set
  * @it: the iterator to advance
  *
  * Advance @it to the next css_set to walk.
  */
-static void css_advance_task_iter(struct css_task_iter *it)
+static void css_task_iter_advance_css_set(struct css_task_iter *it)
 {
         struct list_head *l = it->cset_pos;
         struct cgrp_cset_link *link;
@@ -3809,6 +3809,7 @@ static void css_advance_task_iter(struct css_task_iter *it)
                 l = l->next;
                 if (l == it->cset_head) {
                         it->cset_pos = NULL;
+                        it->task_pos = NULL;
                         return;
                 }
 
@@ -3832,6 +3833,28 @@ static void css_advance_task_iter(struct css_task_iter *it)
         it->mg_tasks_head = &cset->mg_tasks;
 }
 
+static void css_task_iter_advance(struct css_task_iter *it)
+{
+        struct list_head *l = it->task_pos;
+
+        WARN_ON_ONCE(!l);
+
+        /*
+         * Advance iterator to find next entry.  cset->tasks is consumed
+         * first and then ->mg_tasks.  After ->mg_tasks, we move onto the
+         * next cset.
+         */
+        l = l->next;
+
+        if (l == it->tasks_head)
+                l = it->mg_tasks_head->next;
+
+        if (l == it->mg_tasks_head)
+                css_task_iter_advance_css_set(it);
+        else
+                it->task_pos = l;
+}
+
 /**
  * css_task_iter_start - initiate task iteration
  * @css: the css to walk tasks of
@@ -3864,7 +3887,7 @@ void css_task_iter_start(struct cgroup_subsys_state *css,
 
         it->cset_head = it->cset_pos;
 
-        css_advance_task_iter(it);
+        css_task_iter_advance_css_set(it);
 }
 
 /**
@@ -3878,28 +3901,12 @@ void css_task_iter_start(struct cgroup_subsys_state *css,
 struct task_struct *css_task_iter_next(struct css_task_iter *it)
 {
         struct task_struct *res;
-        struct list_head *l = it->task_pos;
 
-        /* If the iterator cg is NULL, we have no tasks */
         if (!it->cset_pos)
                 return NULL;
-        res = list_entry(l, struct task_struct, cg_list);
-
-        /*
-         * Advance iterator to find next entry.  cset->tasks is consumed
-         * first and then ->mg_tasks.  After ->mg_tasks, we move onto the
-         * next cset.
-         */
-        l = l->next;
-
-        if (l == it->tasks_head)
-                l = it->mg_tasks_head->next;
-
-        if (l == it->mg_tasks_head)
-                css_advance_task_iter(it);
-        else
-                it->task_pos = l;
 
+        res = list_entry(it->task_pos, struct task_struct, cg_list);
+        css_task_iter_advance(it);
         return res;
 }