sched, autogroup: Stop going ahead if autogroup is disabled

when autogroup is disable from the beginning,
sched_autogroup_create_attach()
  autogroup_move_group()                    <== 1
    sched_move_task()                       <== 2
      task_move_group_fair()
        set_task_rq()
          task_group()
            autogroup_task_group()

We go the whole path without doing anything useful.

Then stop going further if autogroup is disabled.

But there will be a race window between 1 and 2, in which
sysctl_sched_autogroup_enabled is enabled. This issue
will be toke by following patch.

Signed-off-by: Yong Zhang <yong.zhang0@gmail.com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Mike Galbraith <efault@gmx.de>
LKML-Reference: <1298185696-4403-4-git-send-email-yong.zhang0@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

2 files changed, 9 insertions, 0 deletions

diff --git a/kernel/sched_autogroup.c b/kernel/sched_autogroup.c
index 9fb656283157..137a096ae9d8 100644
--- a/kernel/sched_autogroup.c
+++ b/kernel/sched_autogroup.c
@@ -161,11 +161,15 @@ autogroup_move_group(struct task_struct *p, struct autogroup *ag)
 
         p->signal->autogroup = autogroup_kref_get(ag);
 
+        if (!ACCESS_ONCE(sysctl_sched_autogroup_enabled))
+                goto out;
+
         t = p;
         do {
                 sched_move_task(t);
         } while_each_thread(p, t);
 
+out:
         unlock_task_sighand(p, &flags);
         autogroup_kref_put(prev);
 }
diff --git a/kernel/sched_autogroup.h b/kernel/sched_autogroup.h
index 7b859ffe5dad..05577055cfca 100644
--- a/kernel/sched_autogroup.h
+++ b/kernel/sched_autogroup.h
@@ -1,6 +1,11 @@
 #ifdef CONFIG_SCHED_AUTOGROUP
 
 struct autogroup {
+        /*
+         * reference doesn't mean how many thread attach to this
+         * autogroup now. It just stands for the number of task
+         * could use this autogroup.
+         */
         struct kref             kref;
         struct task_group       *tg;
         struct rw_semaphore     lock;