Re: [PATCH] sched: fix race in schedule

[Sripathi Kodi <sripathik@in.ibm.com>]

On Monday 10 March 2008 23:31, Hiroshi Shimamoto wrote:
> Hi Ingo,
>
> I found a race condition in scheduler.
> The first report is the below;
> http://lkml.org/lkml/2008/2/26/459
>
> It took a bit long time to investigate and I couldn't have much time
> last week. It is hard to reproduce but -rt is little easier because
> it has preemptible spin lock and rcu.
>
> Could you please check the scenario and the patch.
> It will be needed for the stable, too.

Hi,

I can recreate a problem that looks very similar to this on a kernel 
based on 2.6.24.3-rt3. Hiroshi-san's patch seems to fix the problem for 
me.

Ingo, is this patch going to be included in the next -rt patch? I see 
that it is already part of mainline.

Thanks,
Sripathi.


>
> ---
> 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>
> ---
>  kernel/sched.c |   38 ++++++++++++++++----------------------
>  1 files changed, 16 insertions(+), 22 deletions(-)
>
> diff --git a/kernel/sched.c b/kernel/sched.c
> index 52b9867..eedf748 100644
> --- a/kernel/sched.c
> +++ b/kernel/sched.c
> @@ -4268,11 +4268,10 @@ void rt_mutex_setprio(struct task_struct *p,
> int prio) 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;
> @@ -4281,10 +4280,9 @@ void rt_mutex_setprio(struct task_struct *p,
> int prio)
>
>  	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);
>
>  		check_class_changed(rq, p, prev_class, oldprio, running);
> @@ -4581,19 +4579,17 @@ 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);
>
>  		check_class_changed(rq, p, prev_class, oldprio, running);
> @@ -7617,11 +7613,10 @@ void sched_move_task(struct task_struct *tsk)
>  	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_rq(tsk, task_cpu(tsk));
>
> @@ -7630,11 +7625,10 @@ void sched_move_task(struct task_struct *tsk)
>  		tsk->sched_class->moved_group(tsk);
>  #endif
>
> -	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);
> -	}
>
>  	task_rq_unlock(rq, &flags);
>  }