Re: [PATCH RFC tip/core/rcu 7/7] rcu: Quiet RCU-lockdep warningsinvolving interrupt disabling

[Yong Zhang]

On Sat, Dec 03, 2011 at 10:34:42AM -0800, Paul E. McKenney wrote:
> From: Yong Zhang <yong.zhang0@xxxxxxxxx>
> 
> RCU-lockdep will issue warnings given the following use pattern:
> 
> 	rcu_read_lock();
> 	local_irq_disable();
> 	rcu_read_unlock();
> 	local_irq_enable();
> 
> However, this use pattern is legal except for the scheduler's runqueue
> and priority-inheritance locks (and any other locks that the scheduler
> might use during priority-inheritance operations).
> 
> Signed-off-by: Yong Zhang <yong.zhang0@xxxxxxxxx>
> Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
> ---
>  kernel/rcutree_plugin.h |    8 ++++++--
>  1 files changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h
> index 8cd9efe..2020e8a 100644
> --- a/kernel/rcutree_plugin.h
> +++ b/kernel/rcutree_plugin.h
> @@ -401,8 +401,11 @@ static noinline void rcu_read_unlock_special(struct task_struct *t)
>  
>  #ifdef CONFIG_RCU_BOOST
>  		/* Unboost if we were boosted. */
> -		if (rbmp)
> +		if (rbmp) {
> +			local_irq_save(flags);
>  			rt_mutex_unlock(rbmp);
> +			local_irq_restore(flags);
> +		}
>  #endif /* #ifdef CONFIG_RCU_BOOST */
>  
>  		/*
> @@ -1233,9 +1236,10 @@ static int rcu_boost(struct rcu_node *rnp)
>  	lockdep_set_class_and_name(&mtx.wait_lock, &rcu_boost_class,
>  				   "rcu_boost_mutex");
>  	t->rcu_boost_mutex = &mtx;
> -	raw_spin_unlock_irqrestore(&rnp->lock, flags);
> +	raw_spin_unlock(&rnp->lock); /* rrupts remain disabled. */
>  	rt_mutex_lock(&mtx);  /* Side effect: boosts task t's priority. */
>  	rt_mutex_unlock(&mtx);  /* Keep lockdep happy. */

We permit rt_mutex_unlock() to be call with irq disabled,
but rt_mutex_lock() is still not allowed. So this usage
is not legal now.

Sounds we should hold this patch on until a more suitable
way is found.

Thanks,
Yong

> +	local_irq_restore(flags);
>  
>  	return rnp->exp_tasks != NULL || rnp->boost_tasks != NULL;
>  }
> -- 
> 1.7.3.2
> 
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