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Subject[patch 63/76] sched: fix race in schedule()
-stable review patch.  If anyone has any objections, please let us know.
---------------------

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

Fix a hard to trigger crash seen in the -rt kernel that also affects
the vanilla scheduler.

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>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
[chrisw@sous-sol.org: backport to 2.6.24.3]
Signed-off-by: Chris Wright <chrisw@sous-sol.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
---
kernel/sched.c | 36 ++++++++++++++++--------------------
1 file changed, 16 insertions(+), 20 deletions(-)

--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -4028,11 +4028,10 @@ void rt_mutex_setprio(struct task_struct
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;
@@ -4041,9 +4040,9 @@ void rt_mutex_setprio(struct task_struct

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);
/*
* Reschedule if we are currently running on this runqueue and
@@ -4339,18 +4338,17 @@ 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);
/*
* Reschedule if we are currently running on this runqueue and
@@ -7110,19 +7108,17 @@ void sched_move_task(struct task_struct
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_cfs_rq(tsk, task_cpu(tsk));

- 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);
- }

done:
task_rq_unlock(rq, &flags);
--


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