linux-kernel - Re: [PATCH -tip] rcu: local_irq_disable() also delimits RCU

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Date:	Tue, 16 Mar 2010 06:21:56 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Lai Jiangshan <laijs@...fujitsu.com>
Cc:	Ingo Molnar <mingo@...e.hu>, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH -tip] rcu: local_irq_disable() also delimits RCU_SCHED
	read-site critical sections

On Tue, Mar 16, 2010 at 07:09:21PM +0800, Lai Jiangshan wrote:
> It is documented that local_irq_disable() also delimits
> RCU_SCHED read-site critical sections.
> See the document of synchronize_sched() or
> Documentation/RCU/whatisRCU.txt.
> 
> So we have to test irqs_disabled() in rcu_read_lock_sched_held().
> Otherwise rcu-lockdep brings incorrect complaint.

Interesting -- I was under the impression that preempt_count() covered
this as well, due to the following in include/linux/hardirq.h:

	#define PREEMPT_MASK    (__IRQ_MASK(PREEMPT_BITS) << PREEMPT_SHIFT)
	#define SOFTIRQ_MASK    (__IRQ_MASK(SOFTIRQ_BITS) << SOFTIRQ_SHIFT)
	#define HARDIRQ_MASK    (__IRQ_MASK(HARDIRQ_BITS) << HARDIRQ_SHIFT)
	#define NMI_MASK        (__IRQ_MASK(NMI_BITS)     << NMI_SHIFT)

But irqs_disabled() does look to sample the actual interrupt hardware.

So, if there are cases where RCU is used where the interrupt hardware
is disabled, but preempt_count() has not been updated, this patch is
the right thing to do.

							Thanx, Paul

> Signed-off-by: Lai Jiangshan <laijs@...fujitsu.com>
> ---
> diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> index 3024050..2ce5674 100644
> --- a/include/linux/rcupdate.h
> +++ b/include/linux/rcupdate.h
> @@ -160,7 +160,7 @@ static inline int rcu_read_lock_sched_held(void)
>  		return 1;
>  	if (debug_locks)
>  		lockdep_opinion = lock_is_held(&rcu_sched_lock_map);
> -	return lockdep_opinion || preempt_count() != 0;
> +	return lockdep_opinion || preempt_count() != 0 || irqs_disabled();
>  }
>  #else /* #ifdef CONFIG_PREEMPT */
>  static inline int rcu_read_lock_sched_held(void)
> @@ -191,7 +191,7 @@ static inline int rcu_read_lock_bh_held(void)
>  #ifdef CONFIG_PREEMPT
>  static inline int rcu_read_lock_sched_held(void)
>  {
> -	return !rcu_scheduler_active || preempt_count() != 0;
> +	return !rcu_scheduler_active || preempt_count() != 0 || irqs_disabled();
>  }
>  #else /* #ifdef CONFIG_PREEMPT */
>  static inline int rcu_read_lock_sched_held(void)
> 
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