cgroup: move check_for_release() invocation

To trigger release agent when the last task leaves the cgroup,
check_for_release() is called from put_css_set_locked(); however,
css_set being unlinked is being decoupled from task leaving the cgroup
and the correct condition to test is cgroup->nr_populated dropping to
zero which check_for_release() is already updated to test.

This patch moves check_for_release() invocation from
put_css_set_locked() to cgroup_update_populated().

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

kernel/cgroup.c

@@ -623,6 +623,7 @@ static void cgroup_update_populated(struct cgroup *cgrp, bool populated)
 		if (!trigger)
 			break;
 
+		check_for_release(cgrp);
 		cgroup_file_notify(&cgrp->events_file);
 
 		cgrp = cgroup_parent(cgrp);
@@ -686,15 +687,8 @@ static void put_css_set_locked(struct css_set *cset)
 	css_set_count--;
 
 	list_for_each_entry_safe(link, tmp_link, &cset->cgrp_links, cgrp_link) {
-		struct cgroup *cgrp = link->cgrp;
-
 		list_del(&link->cset_link);
 		list_del(&link->cgrp_link);
-
-		/* @cgrp can't go away while we're holding css_set_rwsem */
-		if (list_empty(&cgrp->cset_links))
-			check_for_release(cgrp);
-
 		kfree(link);
 	}