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SubjectRe: [PATCH] sched: fix race in schedule

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 <>

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;
| 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
: |
=> 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 <>
Signed-off-by: Peter Zijlstra <>
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);
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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