linux-kernel - Re: [PATCH] sched/fair: do not announce throttled next buddy in dequeue_task

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Message-ID: <xm26k2hpvzbb.fsf@bsegall-linux.mtv.corp.google.com>
Date:	Thu, 16 Jun 2016 10:06:00 -0700
From:	bsegall@...gle.com
To:	Konstantin Khlebnikov <khlebnikov@...dex-team.ru>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
	stable@...r.kernel.org
Subject: Re: [PATCH] sched/fair: do not announce throttled next buddy in dequeue_task_fair

Konstantin Khlebnikov <khlebnikov@...dex-team.ru> writes:

> Hierarchy could be already throttled at this point. Throttled next
> buddy could trigger null pointer dereference in pick_next_task_fair().
>
> Signed-off-by: Konstantin Khlebnikov <khlebnikov@...dex-team.ru>

Reviewed-by: Ben Segall <bsegall@...gle.com>

> Cc: Stable <stable@...r.kernel.org> # v3.2+
> ---
>  kernel/sched/fair.c |    9 ++++-----
>  1 file changed, 4 insertions(+), 5 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index fe809fe169d2..3c6b038cb734 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -4519,15 +4519,14 @@ static void dequeue_task_fair(struct rq *rq, struct task_struct *p, int flags)
>  
>  		/* Don't dequeue parent if it has other entities besides us */
>  		if (cfs_rq->load.weight) {
> +			/* avoid re-evaluating load for this entity */
> +			se = parent_entity(se);
>  			/*
>  			 * Bias pick_next to pick a task from this cfs_rq, as
>  			 * p is sleeping when it is within its sched_slice.
>  			 */
> -			if (task_sleep && parent_entity(se))
> -				set_next_buddy(parent_entity(se));
> -
> -			/* avoid re-evaluating load for this entity */
> -			se = parent_entity(se);
> +			if (task_sleep && se && !throttled_hierarchy(cfs_rq))
> +				set_next_buddy(se);
>  			break;
>  		}
>  		flags |= DEQUEUE_SLEEP;