linux-kernel - Re: [PATCH v2] sched: fix tsk->pi_lock isn't held when do_set_cpus

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Date:	Fri, 28 Aug 2015 14:33:32 +0800
From:	Boqun Feng <boqun.feng@...il.com>
To:	Wanpeng Li <wanpeng.li@...mail.com>
Cc:	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Sasha Levin <sasha.levin@...cle.com>,
	kernel test robot <ying.huang@...el.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] sched: fix tsk->pi_lock isn't held when
 do_set_cpus_allowed()

Hi Wanpeng,

On Fri, Aug 28, 2015 at 12:02:47PM +0800, Wanpeng Li wrote:
<snip>
> This patch fix it by following the rules for changing task_struct::cpus_allowed
> w/ both pi_lock and rq->lock are held. 
> 
> Reported-by: kernel test robot <ying.huang@...el.com>
> Reported-by: Sasha Levin <sasha.levin@...cle.com>
> Signed-off-by: Wanpeng Li <wanpeng.li@...mail.com>
> ---
>  v1 -> v2:
>   * fix the silly double lock stuff
>   * follow the rules for changing task_struct::cpus_allowed
> 
>  kernel/sched/core.c |   22 ++++++++++++++++++++++
>  1 files changed, 22 insertions(+), 0 deletions(-)
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index b3386c6..8cf87e3 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -5186,6 +5186,27 @@ static void migrate_tasks(struct rq *dead_rq)
>  		BUG_ON(!next);
>  		next->sched_class->put_prev_task(rq, next);
>  
> +		/*
> +		 * Rules for changing task_struct::cpus_allowed are holding
> +		 * both pi_lock and rq->lock, such that holding either
> +		 * stabilizes the mask.
> +		 *
> +		 * Drop rq->lock is not quite as disastrous as it usually is
> +		 * because !cpu_active at this point, which means load-balance
> +		 * will not interfere.
> +		 */
> +		lockdep_unpin_lock(&rq->lock);
> +		raw_spin_unlock(&rq->lock);
> +		raw_spin_lock(&next->pi_lock);
> +		raw_spin_lock(&rq->lock);
> +		lockdep_pin_lock(&rq->lock);
> +		if (!(task_rq(next) == rq && task_on_rq_queued(next))) {
> +			lockdep_unpin_lock(&rq->lock);
> +			raw_spin_unlock(&rq->lock);

Dropping rq->lock here means we will continue the loop without the
rq->lock, right? But we do have a lockdep_pin_lock(&rq->lock) in the
beginning of every iteration. So can we release rq->lock here?

Regards,
Boqun

> +			raw_spin_unlock(&next->pi_lock);
> +			continue;
> +		}
> +
>  		/* Find suitable destination for @next, with force if needed. */
>  		dest_cpu = select_fallback_rq(dead_rq->cpu, next);
>  
> @@ -5196,6 +5217,7 @@ static void migrate_tasks(struct rq *dead_rq)
>  			rq = dead_rq;
>  			raw_spin_lock(&rq->lock);
>  		}
> +		raw_spin_unlock(&next->pi_lock);
>  	}
>  
>  	rq->stop = stop;
> -- 
> 1.7.1
> 
> --
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