2 files changed, 43 insertions, 21 deletions

diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index 8c283a910b91..528e2aed36c3 100644
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -324,10 +324,14 @@ struct css_set {
         struct hlist_node hlist;
 
         /*
-         * List running through all tasks using this cgroup
-         * group. Protected by css_set_lock
+         * Lists running through all tasks using this cgroup group.
+         * mg_tasks lists tasks which belong to this cset but are in the
+         * process of being migrated out or in.  Protected by
+         * css_set_rwsem, but, during migration, once tasks are moved to
+         * mg_tasks, it can be read safely while holding cgroup_mutex.
          */
         struct list_head tasks;
+        struct list_head mg_tasks;
 
         /*
          * List of cgrp_cset_links pointing at cgroups referenced from this
diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 8ab800c7bac0..b80c611ff836 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -644,6 +644,7 @@ static struct css_set *find_css_set(struct css_set *old_cset,
         atomic_set(&cset->refcount, 1);
         INIT_LIST_HEAD(&cset->cgrp_links);
         INIT_LIST_HEAD(&cset->tasks);
+        INIT_LIST_HEAD(&cset->mg_tasks);
         INIT_HLIST_NODE(&cset->hlist);
 
         /* Copy the set of subsystem state objects generated in
@@ -2590,9 +2591,14 @@ static void css_advance_task_iter(struct css_task_iter *it)
                 }
                 link = list_entry(l, struct cgrp_cset_link, cset_link);
                 cset = link->cset;
-        } while (list_empty(&cset->tasks));
+        } while (list_empty(&cset->tasks) && list_empty(&cset->mg_tasks));
+
         it->cset_link = l;
-        it->task = cset->tasks.next;
+
+        if (!list_empty(&cset->tasks))
+                it->task = cset->tasks.next;
+        else
+                it->task = cset->mg_tasks.next;
 }
 
 /**
@@ -2636,24 +2642,29 @@ struct task_struct *css_task_iter_next(struct css_task_iter *it)
 {
         struct task_struct *res;
         struct list_head *l = it->task;
-        struct cgrp_cset_link *link;
+        struct cgrp_cset_link *link = list_entry(it->cset_link,
+                                        struct cgrp_cset_link, cset_link);
 
         /* If the iterator cg is NULL, we have no tasks */
         if (!it->cset_link)
                 return NULL;
         res = list_entry(l, struct task_struct, cg_list);
-        /* Advance iterator to find next entry */
+
+        /*
+         * Advance iterator to find next entry.  cset->tasks is consumed
+         * first and then ->mg_tasks.  After ->mg_tasks, we move onto the
+         * next cset.
+         */
         l = l->next;
-        link = list_entry(it->cset_link, struct cgrp_cset_link, cset_link);
-        if (l == &link->cset->tasks) {
-                /*
-                 * We reached the end of this task list - move on to the
-                 * next cgrp_cset_link.
-                 */
+
+        if (l == &link->cset->tasks)
+                l = link->cset->mg_tasks.next;
+
+        if (l == &link->cset->mg_tasks)
                 css_advance_task_iter(it);
-        } else {
+        else
                 it->task = l;
-        }
+
         return res;
 }
 
@@ -4502,16 +4513,23 @@ static int cgroup_css_links_read(struct seq_file *seq, void *v)
                 struct css_set *cset = link->cset;
                 struct task_struct *task;
                 int count = 0;
+
                 seq_printf(seq, "css_set %p\n", cset);
+
                 list_for_each_entry(task, &cset->tasks, cg_list) {
-                        if (count++ > MAX_TASKS_SHOWN_PER_CSS) {
-                                seq_puts(seq, "  ...\n");
-                                break;
-                        } else {
-                                seq_printf(seq, "  task %d\n",
-                                           task_pid_vnr(task));
-                        }
+                        if (count++ > MAX_TASKS_SHOWN_PER_CSS)
+                                goto overflow;
+                        seq_printf(seq, "  task %d\n", task_pid_vnr(task));
+                }
+
+                list_for_each_entry(task, &cset->mg_tasks, cg_list) {
+                        if (count++ > MAX_TASKS_SHOWN_PER_CSS)
+                                goto overflow;
+                        seq_printf(seq, "  task %d\n", task_pid_vnr(task));
                 }
+                continue;
+        overflow:
+                seq_puts(seq, "  ...\n");
         }
         up_read(&css_set_rwsem);
         return 0;