4 files changed, 33 insertions, 16 deletions

diff --git a/include/linux/init_task.h b/include/linux/init_task.h
index 9e65eff6af3b..b806b821e735 100644
--- a/include/linux/init_task.h
+++ b/include/linux/init_task.h
@@ -123,8 +123,17 @@ extern struct group_info init_groups;
 
 extern struct cred init_cred;
 
+extern struct task_group root_task_group;
+
+#ifdef CONFIG_CGROUP_SCHED
+# define INIT_CGROUP_SCHED(tsk)                                         \
+        .sched_task_group = &root_task_group,
+#else
+# define INIT_CGROUP_SCHED(tsk)
+#endif
+
 #ifdef CONFIG_PERF_EVENTS
-# define INIT_PERF_EVENTS(tsk)                                  \
+# define INIT_PERF_EVENTS(tsk)                                          \
         .perf_event_mutex =                                             \
                  __MUTEX_INITIALIZER(tsk.perf_event_mutex),             \
         .perf_event_list = LIST_HEAD_INIT(tsk.perf_event_list),
@@ -161,6 +170,7 @@ extern struct cred init_cred;
         },                                                              \
         .tasks          = LIST_HEAD_INIT(tsk.tasks),                    \
         INIT_PUSHABLE_TASKS(tsk)                                        \
+        INIT_CGROUP_SCHED(tsk)                                          \
         .ptraced        = LIST_HEAD_INIT(tsk.ptraced),                  \
         .ptrace_entry   = LIST_HEAD_INIT(tsk.ptrace_entry),             \
         .real_parent    = &tsk,                                         \
diff --git a/include/linux/sched.h b/include/linux/sched.h
index bc9952991710..fd9436a3a545 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1245,6 +1245,9 @@ struct task_struct {
         const struct sched_class *sched_class;
         struct sched_entity se;
         struct sched_rt_entity rt;
+#ifdef CONFIG_CGROUP_SCHED
+        struct task_group *sched_task_group;
+#endif
 
 #ifdef CONFIG_PREEMPT_NOTIFIERS
         /* list of struct preempt_notifier: */
@@ -2724,7 +2727,7 @@ extern int sched_group_set_rt_period(struct task_group *tg,
 extern long sched_group_rt_period(struct task_group *tg);
 extern int sched_rt_can_attach(struct task_group *tg, struct task_struct *tsk);
 #endif
-#endif
+#endif /* CONFIG_CGROUP_SCHED */
 
 extern int task_can_switch_user(struct user_struct *up,
                                         struct task_struct *tsk);
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 536b213f0ce5..5d011ef4c0df 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -1096,7 +1096,7 @@ void set_task_cpu(struct task_struct *p, unsigned int new_cpu)
          * a task's CPU. ->pi_lock for waking tasks, rq->lock for runnable tasks.
          *
          * sched_move_task() holds both and thus holding either pins the cgroup,
-         * see set_task_rq().
+         * see task_group().
          *
          * Furthermore, all task_rq users should acquire both locks, see
          * task_rq_lock().
@@ -7658,6 +7658,7 @@ void sched_destroy_group(struct task_group *tg)
  */
 void sched_move_task(struct task_struct *tsk)
 {
+        struct task_group *tg;
         int on_rq, running;
         unsigned long flags;
         struct rq *rq;
@@ -7672,6 +7673,12 @@ void sched_move_task(struct task_struct *tsk)
         if (unlikely(running))
                 tsk->sched_class->put_prev_task(rq, tsk);
 
+        tg = container_of(task_subsys_state_check(tsk, cpu_cgroup_subsys_id,
+                                lockdep_is_held(&tsk->sighand->siglock)),
+                          struct task_group, css);
+        tg = autogroup_task_group(tsk, tg);
+        tsk->sched_task_group = tg;
+
 #ifdef CONFIG_FAIR_GROUP_SCHED
         if (tsk->sched_class->task_move_group)
                 tsk->sched_class->task_move_group(tsk, on_rq);
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 55844f24435a..c35a1a7dd4d6 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -538,22 +538,19 @@ extern int group_balance_cpu(struct sched_group *sg);
 /*
  * Return the group to which this tasks belongs.
  *
- * We use task_subsys_state_check() and extend the RCU verification with
- * pi->lock and rq->lock because cpu_cgroup_attach() holds those locks for each
- * task it moves into the cgroup. Therefore by holding either of those locks,
- * we pin the task to the current cgroup.
+ * We cannot use task_subsys_state() and friends because the cgroup
+ * subsystem changes that value before the cgroup_subsys::attach() method
+ * is called, therefore we cannot pin it and might observe the wrong value.
+ *
+ * The same is true for autogroup's p->signal->autogroup->tg, the autogroup
+ * core changes this before calling sched_move_task().
+ *
+ * Instead we use a 'copy' which is updated from sched_move_task() while
+ * holding both task_struct::pi_lock and rq::lock.
  */
 static inline struct task_group *task_group(struct task_struct *p)
 {
-        struct task_group *tg;
-        struct cgroup_subsys_state *css;
-
-        css = task_subsys_state_check(p, cpu_cgroup_subsys_id,
-                        lockdep_is_held(&p->pi_lock) ||
-                        lockdep_is_held(&task_rq(p)->lock));
-        tg = container_of(css, struct task_group, css);
-
-        return autogroup_task_group(p, tg);
+        return p->sched_task_group;
 }
 
 /* Change a task's cfs_rq and parent entity if it moves across CPUs/groups */