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Subject[PATCH] sched: fix race in schedule
Hi Ingo,

I found a race condition in scheduler.
The first report is the below;
http://lkml.org/lkml/2008/2/26/459

It took a bit long time to investigate and I couldn't have much time last week.
It is hard to reproduce but -rt is little easier because it has preemptible
spin lock and rcu.

Could you please check the scenario and the patch.
It will be needed for the stable, too.

---
From: Hiroshi Shimamoto <h-shimamoto@ct.jp.nec.com>

There is a race condition between schedule() and some dequeue/enqueue
functions; rt_mutex_setprio(), __setscheduler() and sched_move_task().

When scheduling to idle, idle_balance() is called to pull tasks from
other busy processor. It might drop the rq lock.
It means that those 3 functions encounter on_rq=0 and running=1.
The current task should be put when running.

Here is a possible scenario;
CPU0 CPU1
| schedule()
| ->deactivate_task()
| ->idle_balance()
| -->load_balance_newidle()
rt_mutex_setprio() |
| --->double_lock_balance()
*get lock *rel lock
* on_rq=0, ruuning=1 |
* sched_class is changed |
*rel lock *get lock
: |
:
->put_prev_task_rt()
->pick_next_task_fair()
=> panic

The current process of CPU1(P1) is scheduling. Deactivated P1,
and the scheduler looks for another process on other CPU's runqueue
because CPU1 will be idle. idle_balance(), load_balance_newidle()
and double_lock_balance() are called and double_lock_balance() could
drop the rq lock. On the other hand, CPU0 is trying to boost the
priority of P1. The result of boosting only P1's prio and sched_class
are changed to RT. The sched entities of P1 and P1's group are never
put. It makes cfs_rq invalid, because the cfs_rq has curr and no leaf,
but pick_next_task_fair() is called, then the kernel panics.

Signed-off-by: Hiroshi Shimamoto <h-shimamoto@ct.jp.nec.com>
---
kernel/sched.c | 38 ++++++++++++++++----------------------
1 files changed, 16 insertions(+), 22 deletions(-)

diff --git a/kernel/sched.c b/kernel/sched.c
index 52b9867..eedf748 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -4268,11 +4268,10 @@ void rt_mutex_setprio(struct task_struct *p, int prio)
oldprio = p->prio;
on_rq = p->se.on_rq;
running = task_current(rq, p);
- if (on_rq) {
+ if (on_rq)
dequeue_task(rq, p, 0);
- if (running)
- p->sched_class->put_prev_task(rq, p);
- }
+ if (running)
+ p->sched_class->put_prev_task(rq, p);

if (rt_prio(prio))
p->sched_class = &rt_sched_class;
@@ -4281,10 +4280,9 @@ void rt_mutex_setprio(struct task_struct *p, int prio)

p->prio = prio;

+ if (running)
+ p->sched_class->set_curr_task(rq);
if (on_rq) {
- if (running)
- p->sched_class->set_curr_task(rq);
-
enqueue_task(rq, p, 0);

check_class_changed(rq, p, prev_class, oldprio, running);
@@ -4581,19 +4579,17 @@ recheck:
update_rq_clock(rq);
on_rq = p->se.on_rq;
running = task_current(rq, p);
- if (on_rq) {
+ if (on_rq)
deactivate_task(rq, p, 0);
- if (running)
- p->sched_class->put_prev_task(rq, p);
- }
+ if (running)
+ p->sched_class->put_prev_task(rq, p);

oldprio = p->prio;
__setscheduler(rq, p, policy, param->sched_priority);

+ if (running)
+ p->sched_class->set_curr_task(rq);
if (on_rq) {
- if (running)
- p->sched_class->set_curr_task(rq);
-
activate_task(rq, p, 0);

check_class_changed(rq, p, prev_class, oldprio, running);
@@ -7617,11 +7613,10 @@ void sched_move_task(struct task_struct *tsk)
running = task_current(rq, tsk);
on_rq = tsk->se.on_rq;

- if (on_rq) {
+ if (on_rq)
dequeue_task(rq, tsk, 0);
- if (unlikely(running))
- tsk->sched_class->put_prev_task(rq, tsk);
- }
+ if (unlikely(running))
+ tsk->sched_class->put_prev_task(rq, tsk);

set_task_rq(tsk, task_cpu(tsk));

@@ -7630,11 +7625,10 @@ void sched_move_task(struct task_struct *tsk)
tsk->sched_class->moved_group(tsk);
#endif

- if (on_rq) {
- if (unlikely(running))
- tsk->sched_class->set_curr_task(rq);
+ if (unlikely(running))
+ tsk->sched_class->set_curr_task(rq);
+ if (on_rq)
enqueue_task(rq, tsk, 0);
- }

task_rq_unlock(rq, &flags);
}
--
1.5.4.1


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