[PATCH] sched: fix to use invalid sched_class

[Hiroshi Shimamoto <h-shimamoto@ct.jp.nec.com>]

Hi Ingo,

I found an issue about the scheduler.
If you need a test case, please let me know.
Here is a patch.

When using rt_mutex, a NULL pointer dereference is occurred at
enqueue_task_rt. Here is a scenario;
1) there are two threads, the thread A is fair_sched_class and
   thread B is rt_sched_class.
2) Thread A is boosted up to rt_sched_class, because the thread A
   has a rt_mutex lock and the thread B is waiting the lock.
3) At this time, when thread A create a new thread C, the thread
   C has a rt_sched_class.
4) When doing wake_up_new_task() for the thread C, the priority
   of the thread C is out of the RT priority range, because the
   normal priority of thread A is not the RT priority. It makes
   data corruption by overflowing the rt_prio_array.
The new thread C should be fair_sched_class.

The new thread should be valid scheduler class before queuing.
This patch fixes to set the suitable scheduler class.

Signed-off-by: Hiroshi Shimamoto <h-shimamoto@ct.jp.nec.com>
---

diff --git a/kernel/sched.c b/kernel/sched.c
index deeb1f8..b0c582b 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -1682,6 +1682,11 @@ void fastcall wake_up_new_task(struct task_struct *p, unsigned long clone_flags)
 
 	p->prio = effective_prio(p);
 
+	if (rt_prio(p->prio))
+		p->sched_class = &rt_sched_class;
+	else
+		p->sched_class = &fair_sched_class;
+
 	if (!p->sched_class->task_new || !sysctl_sched_child_runs_first ||
 			(clone_flags & CLONE_VM) || task_cpu(p) != this_cpu ||
 			!current->se.on_rq) {