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SubjectRe: [PATCH] sched: fix race in schedule
Peter Zijlstra wrote:
> 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:

thanks for working, I'm OK, and will test it soon.
IIRC, it came from the group scheduling, .23 probably doesn't have this issue.

thanks.

>
> ---
> 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>
> Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
> CC: stable@kernel.org
> ---
> 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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