[tip:sched/core] sched, autogroup: Stop going ahead if autogroup is disabled

[tip-bot for Yong Zhang <yong.zhang0@gmail.com>]

Commit-ID:  800d4d30c8f20bd728e5741a3b77c4859a613f7c
Gitweb:     http://git.kernel.org/tip/800d4d30c8f20bd728e5741a3b77c4859a613f7c
Author:     Yong Zhang <yong.zhang0@gmail.com>
AuthorDate: Sun, 20 Feb 2011 15:08:14 +0800
Committer:  Ingo Molnar <mingo@elte.hu>
CommitDate: Wed, 23 Feb 2011 11:33:59 +0100

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>
---
 kernel/sched_autogroup.c |    4 ++++
 kernel/sched_autogroup.h |    5 +++++
 2 files changed, 9 insertions(+), 0 deletions(-)

diff --git a/kernel/sched_autogroup.c b/kernel/sched_autogroup.c
index 9fb6562..137a096 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 7b859ff..0557705 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;