Re: [PATCH] sched: fix race in schedule

[Peter Zijlstra]

On Mon, 2008-03-10 at 13:01 -0700, Hiroshi Shimamoto wrote:

> thanks, your patch looks nice to me.
> I had focused setprio, on_rq=0 and running=1 situation, it makes me to
> fix these functions.
> But one point, I've just noticed. I'm not sure on same situation against
> sched_rt. I think the pre_schedule() of rt has chance to drop rq lock.
> Is it OK?

Ah, you are quite right, that'll teach me to rush out a patch just
because dinner is ready :-). 

How about we submit the following patch for mainline and CC -stable to
fix .23 and .24:

---
From: Hiroshi Shimamoto <h-shimamoto@xxxxxxxxxxxxx>

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@xxxxxxxxxxxxx>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
CC: stable@xxxxxxxxxx
---
 kernel/sched.c |    8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

Index: linux-2.6-2/kernel/sched.c
===================================================================
--- linux-2.6-2.orig/kernel/sched.c
+++ linux-2.6-2/kernel/sched.c
@@ -4062,6 +4062,13 @@ need_resched_nonpreemptible:
 		switch_count = &prev->nvcsw;
 	}
 
+	/*
+	 * ->pre_schedule() and idle_balance() can release the rq->lock so we
+	 * have to call ->put_prev_task() before we do the balancing calls,
+	 * otherwise its possible to see the rq in an inconsistent state.
+	 */
+	prev->sched_class->put_prev_task(rq, prev);
+
 #ifdef CONFIG_SMP
 	if (prev->sched_class->pre_schedule)
 		prev->sched_class->pre_schedule(rq, prev);
@@ -4070,7 +4077,6 @@ need_resched_nonpreemptible:
 	if (unlikely(!rq->nr_running))
 		idle_balance(cpu, rq);
 
-	prev->sched_class->put_prev_task(rq, prev);
 	next = pick_next_task(rq, prev);
 
 	sched_info_switch(prev, next);


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