cgroup: implement __cgroup_task_count() helper

The helper is identical to the existing cgroup_task_count()
except it doesn't take the css_set_lock by itself, assuming
that the caller does.

Also, move cgroup_task_count() implementation into
kernel/cgroup/cgroup.c, as there is nothing specific to cgroup v1.
Signed-off-by: Roman Gushchin <guro@fb.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: kernel-team@fb.com

kernel/cgroup/cgroup-internal.h

@@ -240,6 +240,7 @@ int cgroup_rmdir(struct kernfs_node *kn);
 int cgroup_show_path(struct seq_file *sf, struct kernfs_node *kf_node,
 		     struct kernfs_root *kf_root);
 
+int __cgroup_task_count(const struct cgroup *cgrp);
 int cgroup_task_count(const struct cgroup *cgrp);
 
 /*

kernel/cgroup/cgroup-v1.c

@@ -342,22 +342,6 @@ static struct cgroup_pidlist *cgroup_pidlist_find_create(struct cgroup *cgrp,
 	return l;
 }
 
-/**
- * cgroup_task_count - count the number of tasks in a cgroup.
- * @cgrp: the cgroup in question
- */
-int cgroup_task_count(const struct cgroup *cgrp)
-{
-	int count = 0;
-	struct cgrp_cset_link *link;
-
-	spin_lock_irq(&css_set_lock);
-	list_for_each_entry(link, &cgrp->cset_links, cset_link)
-		count += link->cset->nr_tasks;
-	spin_unlock_irq(&css_set_lock);
-	return count;
-}
-
 /*
  * Load a cgroup's pidarray with either procs' tgids or tasks' pids
  */

kernel/cgroup/cgroup.c

@@ -593,6 +593,39 @@ static void cgroup_get_live(struct cgroup *cgrp)
 	css_get(&cgrp->self);
 }
 
+/**
+ * __cgroup_task_count - count the number of tasks in a cgroup. The caller
+ * is responsible for taking the css_set_lock.
+ * @cgrp: the cgroup in question
+ */
+int __cgroup_task_count(const struct cgroup *cgrp)
+{
+	int count = 0;
+	struct cgrp_cset_link *link;
+
+	lockdep_assert_held(&css_set_lock);
+
+	list_for_each_entry(link, &cgrp->cset_links, cset_link)
+		count += link->cset->nr_tasks;
+
+	return count;
+}
+
+/**
+ * cgroup_task_count - count the number of tasks in a cgroup.
+ * @cgrp: the cgroup in question
+ */
+int cgroup_task_count(const struct cgroup *cgrp)
+{
+	int count;
+
+	spin_lock_irq(&css_set_lock);
+	count = __cgroup_task_count(cgrp);
+	spin_unlock_irq(&css_set_lock);
+
+	return count;
+}
+
 struct cgroup_subsys_state *of_css(struct kernfs_open_file *of)
 {
 	struct cgroup *cgrp = of->kn->parent->priv;