Merge branch 'for-4.13-fixes' of git://git.kernel.org/pub/scm/linux/kernel/git/tj/cgroup

Pull cgroup fixes from Tejun Heo:
 "Several cgroup bug fixes.

   - cgroup core was calling a migration callback on empty migrations,
     which could make cpuset crash.

   - There was a very subtle bug where the controller interface files
     aren't created directly when cgroup2 is mounted. Because later
     operations create them, this bug didn't get noticed earlier.

   - Failed writes to cgroup.subtree_control were incorrectly returning
     zero"

* 'for-4.13-fixes' of git://git.kernel.org/pub/scm/linux/kernel/git/tj/cgroup:
  cgroup: fix error return value from cgroup_subtree_control()
  cgroup: create dfl_root files on subsys registration
  cgroup: don't call migration methods if there are no tasks to migrate

2 files changed, 40 insertions, 29 deletions

diff --git a/kernel/cgroup/cgroup-internal.h b/kernel/cgroup/cgroup-internal.h
index 793565c05742..8b4c3c2f2509 100644
--- a/kernel/cgroup/cgroup-internal.h
+++ b/kernel/cgroup/cgroup-internal.h
@@ -33,6 +33,9 @@ struct cgroup_taskset {
         struct list_head        src_csets;
         struct list_head        dst_csets;
 
+        /* the number of tasks in the set */
+        int                     nr_tasks;
+
         /* the subsys currently being processed */
         int                     ssid;
 
diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
index 620794a20a33..df2e0f14a95d 100644
--- a/kernel/cgroup/cgroup.c
+++ b/kernel/cgroup/cgroup.c
@@ -2006,6 +2006,8 @@ static void cgroup_migrate_add_task(struct task_struct *task,
         if (!cset->mg_src_cgrp)
                 return;
 
+        mgctx->tset.nr_tasks++;
+
         list_move_tail(&task->cg_list, &cset->mg_tasks);
         if (list_empty(&cset->mg_node))
                 list_add_tail(&cset->mg_node,
@@ -2094,21 +2096,19 @@ static int cgroup_migrate_execute(struct cgroup_mgctx *mgctx)
         struct css_set *cset, *tmp_cset;
         int ssid, failed_ssid, ret;
 
-        /* methods shouldn't be called if no task is actually migrating */
-        if (list_empty(&tset->src_csets))
-                return 0;
-
         /* check that we can legitimately attach to the cgroup */
-        do_each_subsys_mask(ss, ssid, mgctx->ss_mask) {
-                if (ss->can_attach) {
-                        tset->ssid = ssid;
-                        ret = ss->can_attach(tset);
-                        if (ret) {
-                                failed_ssid = ssid;
-                                goto out_cancel_attach;
+        if (tset->nr_tasks) {
+                do_each_subsys_mask(ss, ssid, mgctx->ss_mask) {
+                        if (ss->can_attach) {
+                                tset->ssid = ssid;
+                                ret = ss->can_attach(tset);
+                                if (ret) {
+                                        failed_ssid = ssid;
+                                        goto out_cancel_attach;
+                                }
                         }
-                }
-        } while_each_subsys_mask();
+                } while_each_subsys_mask();
+        }
 
         /*
          * Now that we're guaranteed success, proceed to move all tasks to
@@ -2137,25 +2137,29 @@ static int cgroup_migrate_execute(struct cgroup_mgctx *mgctx)
          */
         tset->csets = &tset->dst_csets;
 
-        do_each_subsys_mask(ss, ssid, mgctx->ss_mask) {
-                if (ss->attach) {
-                        tset->ssid = ssid;
-                        ss->attach(tset);
-                }
-        } while_each_subsys_mask();
+        if (tset->nr_tasks) {
+                do_each_subsys_mask(ss, ssid, mgctx->ss_mask) {
+                        if (ss->attach) {
+                                tset->ssid = ssid;
+                                ss->attach(tset);
+                        }
+                } while_each_subsys_mask();
+        }
 
         ret = 0;
         goto out_release_tset;
 
 out_cancel_attach:
-        do_each_subsys_mask(ss, ssid, mgctx->ss_mask) {
-                if (ssid == failed_ssid)
-                        break;
-                if (ss->cancel_attach) {
-                        tset->ssid = ssid;
-                        ss->cancel_attach(tset);
-                }
-        } while_each_subsys_mask();
+        if (tset->nr_tasks) {
+                do_each_subsys_mask(ss, ssid, mgctx->ss_mask) {
+                        if (ssid == failed_ssid)
+                                break;
+                        if (ss->cancel_attach) {
+                                tset->ssid = ssid;
+                                ss->cancel_attach(tset);
+                        }
+                } while_each_subsys_mask();
+        }
 out_release_tset:
         spin_lock_irq(&css_set_lock);
         list_splice_init(&tset->dst_csets, &tset->src_csets);
@@ -2997,11 +3001,11 @@ static ssize_t cgroup_subtree_control_write(struct kernfs_open_file *of,
         cgrp->subtree_control &= ~disable;
 
         ret = cgroup_apply_control(cgrp);
-
         cgroup_finalize_control(cgrp, ret);
+        if (ret)
+                goto out_unlock;
 
         kernfs_activate(cgrp->kn);
-        ret = 0;
 out_unlock:
         cgroup_kn_unlock(of->kn);
         return ret ?: nbytes;
@@ -4669,6 +4673,10 @@ int __init cgroup_init(void)
 
                 if (ss->bind)
                         ss->bind(init_css_set.subsys[ssid]);
+
+                mutex_lock(&cgroup_mutex);
+                css_populate_dir(init_css_set.subsys[ssid]);
+                mutex_unlock(&cgroup_mutex);
         }
 
         /* init_css_set.subsys[] has been updated, re-hash */