1 files changed, 19 insertions, 5 deletions

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 37b6d534b0ca..98a8045e2149 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -1761,7 +1761,14 @@ static void cgroup_task_migrate(struct cgroup *old_cgrp,
 
         get_css_set(new_cset);
         rcu_assign_pointer(tsk->cgroups, new_cset);
-        list_move(&tsk->cg_list, &new_cset->mg_tasks);
+
+        /*
+         * Use move_tail so that cgroup_taskset_first() still returns the
+         * leader after migration.  This works because cgroup_migrate()
+         * ensures that the dst_cset of the leader is the first on the
+         * tset's dst_csets list.
+         */
+        list_move_tail(&tsk->cg_list, &new_cset->mg_tasks);
 
         /*
          * We just gained a reference on old_cset by taking it from the
@@ -1936,9 +1943,16 @@ static int cgroup_migrate(struct cgroup *cgrp, struct task_struct *leader,
                 if (!cset->mg_src_cgrp)
                         goto next;
 
-                list_move(&task->cg_list, &cset->mg_tasks);
-                list_move(&cset->mg_node, &tset.src_csets);
-                list_move(&cset->mg_dst_cset->mg_node, &tset.dst_csets);
+                /*
+                 * cgroup_taskset_first() must always return the leader.
+                 * Take care to avoid disturbing the ordering.
+                 */
+                list_move_tail(&task->cg_list, &cset->mg_tasks);
+                if (list_empty(&cset->mg_node))
+                        list_add_tail(&cset->mg_node, &tset.src_csets);
+                if (list_empty(&cset->mg_dst_cset->mg_node))
+                        list_move_tail(&cset->mg_dst_cset->mg_node,
+                                       &tset.dst_csets);
         next:
                 if (!threadgroup)
                         break;
@@ -1999,7 +2013,7 @@ out_release_tset:
         down_write(&css_set_rwsem);
         list_splice_init(&tset.dst_csets, &tset.src_csets);
         list_for_each_entry_safe(cset, tmp_cset, &tset.src_csets, mg_node) {
-                list_splice_init(&cset->mg_tasks, &cset->tasks);
+                list_splice_tail_init(&cset->mg_tasks, &cset->tasks);
                 list_del_init(&cset->mg_node);
         }
         up_write(&css_set_rwsem);