sched/autogroup: Fix autogroup_move_group() to never skip sched_move_task()

[mirror_ubuntu-zesty-kernel.git] / kernel / sched / auto_group.c

diff --git a/kernel/sched/auto_group.c b/kernel/sched/auto_group.c
index a5d966cb889175d5b196283341b1cead20f20603..ad2b19ad6ca0264fc4b601b138ddcb5c2c7d8cfc 100644 (file)
--- a/kernel/sched/auto_group.c
+++ b/kernel/sched/auto_group.c
@@ -111,14 +111,11 @@ 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().
         */
-       if (p->flags & PF_EXITING)
-               return false;
-
        return true;
 }
 
@@ -138,13 +135,17 @@ 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.
+        */
        for_each_thread(p, t)
                sched_move_task(t);
-out:
+
        unlock_task_sighand(p, &flags);
        autogroup_kref_put(prev);
 }