Re: [PATCH v6 4/5] sched/core: Prevent race condition between cpuset and __sched_setscheduler()

[Peter Zijlstra <peterz@infradead.org>]

On Thu, Jan 17, 2019 at 09:47:38AM +0100, Juri Lelli wrote:
> No synchronisation mechanism exists between the cpuset subsystem and calls
> to function __sched_setscheduler(). As such, it is possible that new root
> domains are created on the cpuset side while a deadline acceptance test
> is carried out in __sched_setscheduler(), leading to a potential oversell
> of CPU bandwidth.
> 
> Grab callback_lock from core scheduler, so to prevent situations such as
> the one described above from happening.

> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index f5263383170e..d928a42b8852 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -4224,6 +4224,13 @@ static int __sched_setscheduler(struct task_struct *p,
>  	rq = task_rq_lock(p, &rf);
>  	update_rq_clock(rq);
>  
> +	/*
> +	 * Make sure we don't race with the cpuset subsystem where root
> +	 * domains can be rebuilt or modified while operations like DL
> +	 * admission checks are carried out.
> +	 */
> +	cpuset_read_only_lock();
> +
>  	/*
>  	 * Changing the policy of the stop threads its a very bad idea:
>  	 */
> @@ -4285,6 +4292,7 @@ static int __sched_setscheduler(struct task_struct *p,
>  	/* Re-check policy now with rq lock held: */
>  	if (unlikely(oldpolicy != -1 && oldpolicy != p->policy)) {
>  		policy = oldpolicy = -1;
> +		cpuset_read_only_unlock();
>  		task_rq_unlock(rq, p, &rf);
>  		goto recheck;
>  	}
> @@ -4342,6 +4350,7 @@ static int __sched_setscheduler(struct task_struct *p,
>  
>  	/* Avoid rq from going away on us: */
>  	preempt_disable();
> +	cpuset_read_only_unlock();
>  	task_rq_unlock(rq, p, &rf);
>  
>  	if (pi)
> @@ -4354,6 +4363,7 @@ static int __sched_setscheduler(struct task_struct *p,
>  	return 0;
>  
>  unlock:
> +	cpuset_read_only_unlock();
>  	task_rq_unlock(rq, p, &rf);
>  	return retval;
>  }

Why take callback_lock inside rq->lock and not the other way around?
AFAICT there is no pre-existing order so we can pick one here.