linux-kernel - Re: [BUG -tip/sched] INFO: suspicious RCU usage

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Date:	Thu, 8 Dec 2011 09:55:41 +0800
From:	Yong Zhang <yong.zhang0@...il.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	linux-kernel@...r.kernel.org, mingo@...e.hu,
	Suresh Siddha <suresh.b.siddha@...el.com>
Subject: Re: [BUG -tip/sched] INFO: suspicious RCU usage

On Wed, Dec 07, 2011 at 02:32:08PM +0100, Peter Zijlstra wrote:
> I made that
> 
> ---
> Subject: sched, nohz: Fix missing RCU read lock
> From: Peter Zijlstra <a.p.zijlstra@...llo.nl>
> Date: Wed Dec 07 14:28:44 CET 2011
> 
> Yong Zhang reported:
> 
> > [ INFO: suspicious RCU usage. ]
> > kernel/sched/fair.c:5091 suspicious rcu_dereference_check() usage!
> 
> This is due to the sched_domain stuff being RCU protected and commit
> 0b005cf5 ("sched, nohz: Implement sched group, domain
> aware nohz idle load balancing") overlooking this fact.
> 
> The sd variable only lives inside the for_each_domain() block, so we
> only need to wrap that.
> 

Works for me.
Tested-by: Yong Zhang <yong.zhang0@...il.com>

Thanks,
Yong

> Cc: Suresh Siddha <suresh.b.siddha@...el.com>
> Reported-by: Yong Zhang <yong.zhang0@...il.com>
> Signed-off-by: Peter Zijlstra <a.p.zijlstra@...llo.nl>
> Link: http://lkml.kernel.org/n/tip-gt616u9aetdzgb1oc4m6uydr@git.kernel.org
> ---
> Index: linux-2.6/kernel/sched/fair.c
> ===================================================================
> --- linux-2.6.orig/kernel/sched/fair.c
> +++ linux-2.6/kernel/sched/fair.c
> @@ -5088,23 +5088,28 @@ static inline int nohz_kick_needed(struc
>  	if (rq->nr_running >= 2)
>  		goto need_kick;
>  
> +	rcu_read_lock();
>  	for_each_domain(cpu, sd) {
>  		struct sched_group *sg = sd->groups;
>  		struct sched_group_power *sgp = sg->sgp;
>  		int nr_busy = atomic_read(&sgp->nr_busy_cpus);
>  
>  		if (sd->flags & SD_SHARE_PKG_RESOURCES && nr_busy > 1)
> -			goto need_kick;
> +			goto need_kick_unlock;
>  
>  		if (sd->flags & SD_ASYM_PACKING && nr_busy != sg->group_weight
>  		    && (cpumask_first_and(nohz.idle_cpus_mask,
>  					  sched_domain_span(sd)) < cpu))
> -			goto need_kick;
> +			goto need_kick_unlock;
>  
>  		if (!(sd->flags & (SD_SHARE_PKG_RESOURCES | SD_ASYM_PACKING)))
>  			break;
>  	}
> +	rcu_read_unlock();
>  	return 0;
> +
> +need_kick_unlock:
> +	rcu_read_unlock();
>  need_kick:
>  	return 1;
>  }
> 
> --
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