cgroup: replace css_set walking populated test with testing cgrp->nr_populated_csets

Implement trivial cgroup_has_tasks() which tests whether
cgrp->nr_populated_csets is zero and replace the explicit local
populated test in cgroup_subtree_control().  This simplifies the code
and cgroup_has_tasks() will be used in more places later.
Signed-off-by: Tejun Heo <tj@kernel.org>

kernel/cgroup/cgroup.c

@@ -325,6 +325,11 @@ static struct cgroup *cgroup_parent(struct cgroup *cgrp)
 	return NULL;
 }
 
+static bool cgroup_has_tasks(struct cgroup *cgrp)
+{
+	return cgrp->nr_populated_csets;
+}
+
 /* subsystems visibly enabled on a cgroup */
 static u16 cgroup_control(struct cgroup *cgrp)
 {
@@ -2971,28 +2976,9 @@ static ssize_t cgroup_subtree_control_write(struct kernfs_open_file *of,
 	 * Except for the root, subtree_control must be zero for a cgroup
 	 * with tasks so that child cgroups don't compete against tasks.
 	 */
-	if (enable && cgroup_parent(cgrp)) {
-		struct cgrp_cset_link *link;
-
-		/*
-		 * Because namespaces pin csets too, @cgrp->cset_links
-		 * might not be empty even when @cgrp is empty.  Walk and
-		 * verify each cset.
-		 */
-		spin_lock_irq(&css_set_lock);
-
-		ret = 0;
-		list_for_each_entry(link, &cgrp->cset_links, cset_link) {
-			if (css_set_populated(link->cset)) {
-				ret = -EBUSY;
-				break;
-			}
-		}
-
-		spin_unlock_irq(&css_set_lock);
-
-		if (ret)
-			goto out_unlock;
+	if (enable && cgroup_parent(cgrp) && cgroup_has_tasks(cgrp)) {
+		ret = -EBUSY;
+		goto out_unlock;
 	}
 
 	/* save and update control masks and prepare csses */