cgroup: Clean up css_set task traversal

css_task_iter stores pointer to head of each iterable list, this dates
back to commit 0f0a2b4f ("cgroup: reorganize css_task_iter") when we
did not store cur_cset. Let us utilize list heads directly in cur_cset
and streamline css_task_iter_advance_css_set a bit. This is no
intentional function change.
Signed-off-by: Michal Koutný <mkoutny@suse.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

include/linux/cgroup.h

@@ -58,9 +58,6 @@ struct css_task_iter {
 	struct list_head		*tcset_head;
 
 	struct list_head		*task_pos;
-	struct list_head		*tasks_head;
-	struct list_head		*mg_tasks_head;
-	struct list_head		*dying_tasks_head;
 
 	struct list_head		*cur_tasks_head;
 	struct css_set			*cur_cset;

kernel/cgroup/cgroup.c

@@ -4391,29 +4391,24 @@ static void css_task_iter_advance_css_set(struct css_task_iter *it)
 
 	lockdep_assert_held(&css_set_lock);
 
-	/* Advance to the next non-empty css_set */
-	do {
-		cset = css_task_iter_next_css_set(it);
-		if (!cset) {
-			it->task_pos = NULL;
-			return;
+	/* Advance to the next non-empty css_set and find first non-empty tasks list*/
+	while ((cset = css_task_iter_next_css_set(it))) {
+		if (!list_empty(&cset->tasks)) {
+			it->cur_tasks_head = &cset->tasks;
+			break;
+		} else if (!list_empty(&cset->mg_tasks)) {
+			it->cur_tasks_head = &cset->mg_tasks;
+			break;
+		} else if (!list_empty(&cset->dying_tasks)) {
+			it->cur_tasks_head = &cset->dying_tasks;
+			break;
 		}
-	} while (!css_set_populated(cset) && list_empty(&cset->dying_tasks));
-
-	if (!list_empty(&cset->tasks)) {
-		it->task_pos = cset->tasks.next;
-		it->cur_tasks_head = &cset->tasks;
-	} else if (!list_empty(&cset->mg_tasks)) {
-		it->task_pos = cset->mg_tasks.next;
-		it->cur_tasks_head = &cset->mg_tasks;
-	} else {
-		it->task_pos = cset->dying_tasks.next;
-		it->cur_tasks_head = &cset->dying_tasks;
 	}
-
-	it->tasks_head = &cset->tasks;
-	it->mg_tasks_head = &cset->mg_tasks;
-	it->dying_tasks_head = &cset->dying_tasks;
+	if (!cset) {
+		it->task_pos = NULL;
+		return;
+	}
+	it->task_pos = it->cur_tasks_head->next;
 
 	/*
 	 * We don't keep css_sets locked across iteration steps and thus
@@ -4458,24 +4453,24 @@ static void css_task_iter_advance(struct css_task_iter *it)
 repeat:
 	if (it->task_pos) {
 		/*
-		 * Advance iterator to find next entry.  cset->tasks is
-		 * consumed first and then ->mg_tasks.  After ->mg_tasks,
-		 * we move onto the next cset.
+		 * Advance iterator to find next entry. We go through cset
+		 * tasks, mg_tasks and dying_tasks, when consumed we move onto
+		 * the next cset.
 		 */
 		if (it->flags & CSS_TASK_ITER_SKIPPED)
 			it->flags &= ~CSS_TASK_ITER_SKIPPED;
 		else
 			it->task_pos = it->task_pos->next;
 
-		if (it->task_pos == it->tasks_head) {
-			it->task_pos = it->mg_tasks_head->next;
-			it->cur_tasks_head = it->mg_tasks_head;
+		if (it->task_pos == &it->cur_cset->tasks) {
+			it->cur_tasks_head = &it->cur_cset->mg_tasks;
+			it->task_pos = it->cur_tasks_head->next;
 		}
-		if (it->task_pos == it->mg_tasks_head) {
-			it->task_pos = it->dying_tasks_head->next;
-			it->cur_tasks_head = it->dying_tasks_head;
+		if (it->task_pos == &it->cur_cset->mg_tasks) {
+			it->cur_tasks_head = &it->cur_cset->dying_tasks;
+			it->task_pos = it->cur_tasks_head->next;
 		}
-		if (it->task_pos == it->dying_tasks_head)
+		if (it->task_pos == &it->cur_cset->dying_tasks)
 			css_task_iter_advance_css_set(it);
 	} else {
 		/* called from start, proceed to the first cset */
@@ -4493,12 +4488,12 @@ static void css_task_iter_advance(struct css_task_iter *it)
 			goto repeat;
 
 		/* and dying leaders w/o live member threads */
-		if (it->cur_tasks_head == it->dying_tasks_head &&
+		if (it->cur_tasks_head == &it->cur_cset->dying_tasks &&
 		    !atomic_read(&task->signal->live))
 			goto repeat;
 	} else {
 		/* skip all dying ones */
-		if (it->cur_tasks_head == it->dying_tasks_head)
+		if (it->cur_tasks_head == &it->cur_cset->dying_tasks)
 			goto repeat;
 	}
 }