Re: [PATCH] sched: fix race in schedule

[Hiroshi Shimamoto <h-shimamoto@ct.jp.nec.com>]

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);
> 
>