Merge branch 'sched-urgent-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip

Pull scheduler fixes from Ingo Molnar:
 "Two fixes for autogroup scheduling, for races when turning the feature
  on/off via /proc/sys/kernel/sched_autogroup_enabled"

* 'sched-urgent-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip:
  sched/autogroup: Do not use autogroup->tg in zombie threads
  sched/autogroup: Fix autogroup_move_group() to never skip sched_move_task()

3 files changed, 31 insertions, 8 deletions

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 348f51b0ec92..e9c009dc3a4a 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -2567,6 +2567,7 @@ extern void sched_autogroup_create_attach(struct task_struct *p);
 extern void sched_autogroup_detach(struct task_struct *p);
 extern void sched_autogroup_fork(struct signal_struct *sig);
 extern void sched_autogroup_exit(struct signal_struct *sig);
+extern void sched_autogroup_exit_task(struct task_struct *p);
 #ifdef CONFIG_PROC_FS
 extern void proc_sched_autogroup_show_task(struct task_struct *p, struct seq_file *m);
 extern int proc_sched_autogroup_set_nice(struct task_struct *p, int nice);
@@ -2576,6 +2577,7 @@ static inline void sched_autogroup_create_attach(struct task_struct *p) { }
 static inline void sched_autogroup_detach(struct task_struct *p) { }
 static inline void sched_autogroup_fork(struct signal_struct *sig) { }
 static inline void sched_autogroup_exit(struct signal_struct *sig) { }
+static inline void sched_autogroup_exit_task(struct task_struct *p) { }
 #endif
 
 extern int yield_to(struct task_struct *p, bool preempt);
diff --git a/kernel/exit.c b/kernel/exit.c
index 9d68c45ebbe3..3076f3089919 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -836,6 +836,7 @@ void __noreturn do_exit(long code)
          */
         perf_event_exit_task(tsk);
 
+        sched_autogroup_exit_task(tsk);
         cgroup_exit(tsk);
 
         /*
diff --git a/kernel/sched/auto_group.c b/kernel/sched/auto_group.c
index a5d966cb8891..f1c8fd566246 100644
--- a/kernel/sched/auto_group.c
+++ b/kernel/sched/auto_group.c
@@ -111,10 +111,13 @@ bool task_wants_autogroup(struct task_struct *p, struct task_group *tg)
 {
         if (tg != &root_task_group)
                 return false;
-
         /*
-         * We can only assume the task group can't go away on us if
-         * autogroup_move_group() can see us on ->thread_group list.
+         * If we race with autogroup_move_group() the caller can use the old
+         * value of signal->autogroup but in this case sched_move_task() will
+         * be called again before autogroup_kref_put().
+         *
+         * However, there is no way sched_autogroup_exit_task() could tell us
+         * to avoid autogroup->tg, so we abuse PF_EXITING flag for this case.
          */
         if (p->flags & PF_EXITING)
                 return false;
@@ -122,6 +125,16 @@ bool task_wants_autogroup(struct task_struct *p, struct task_group *tg)
         return true;
 }
 
+void sched_autogroup_exit_task(struct task_struct *p)
+{
+        /*
+         * We are going to call exit_notify() and autogroup_move_group() can't
+         * see this thread after that: we can no longer use signal->autogroup.
+         * See the PF_EXITING check in task_wants_autogroup().
+         */
+        sched_move_task(p);
+}
+
 static void
 autogroup_move_group(struct task_struct *p, struct autogroup *ag)
 {
@@ -138,13 +151,20 @@ autogroup_move_group(struct task_struct *p, struct autogroup *ag)
         }
 
         p->signal->autogroup = autogroup_kref_get(ag);
-
-        if (!READ_ONCE(sysctl_sched_autogroup_enabled))
-                goto out;
-
+        /*
+         * We can't avoid sched_move_task() after we changed signal->autogroup,
+         * this process can already run with task_group() == prev->tg or we can
+         * race with cgroup code which can read autogroup = prev under rq->lock.
+         * In the latter case for_each_thread() can not miss a migrating thread,
+         * cpu_cgroup_attach() must not be possible after cgroup_exit() and it
+         * can't be removed from thread list, we hold ->siglock.
+         *
+         * If an exiting thread was already removed from thread list we rely on
+         * sched_autogroup_exit_task().
+         */
         for_each_thread(p, t)
                 sched_move_task(t);
-out:
+
         unlock_task_sighand(p, &flags);
         autogroup_kref_put(prev);
 }