linux-kernel - Re: [PATCH RT 1/4] rcu: Acquire RCU lock when disabling BHs

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Date:   Thu, 20 Jun 2019 13:53:52 -0700
From:   "Paul E. McKenney" <paulmck@...ux.ibm.com>
To:     Scott Wood <swood@...hat.com>
Cc:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Juri Lelli <juri.lelli@...hat.com>,
        Clark Williams <williams@...hat.com>,
        linux-rt-users@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH RT 1/4] rcu: Acquire RCU lock when disabling BHs

On Tue, Jun 18, 2019 at 08:19:05PM -0500, Scott Wood wrote:
> A plain local_bh_disable() is documented as creating an RCU critical
> section, and (at least) rcutorture expects this to be the case.  However,
> in_softirq() doesn't block a grace period on PREEMPT_RT, since RCU checks
> preempt_count() directly.  Even if RCU were changed to check
> in_softirq(), that wouldn't allow blocked BH disablers to be boosted.
> 
> Fix this by calling rcu_read_lock() from local_bh_disable(), and update
> rcu_read_lock_bh_held() accordingly.
> 
> Signed-off-by: Scott Wood <swood@...hat.com>
> ---
>  include/linux/rcupdate.h |  4 ++++
>  kernel/rcu/update.c      |  4 ++++
>  kernel/softirq.c         | 12 +++++++++---
>  3 files changed, 17 insertions(+), 3 deletions(-)
> 
> diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> index fb267bc04fdf..aca4e5e25ace 100644
> --- a/include/linux/rcupdate.h
> +++ b/include/linux/rcupdate.h
> @@ -637,10 +637,12 @@ static inline void rcu_read_unlock(void)
>  static inline void rcu_read_lock_bh(void)
>  {
>  	local_bh_disable();
> +#ifndef CONFIG_PREEMPT_RT_FULL

How about this instead?

	if (IS_ENABLED(CONFIG_PREEMPT_RT_FULL))
		return;

And similarly below.

>  	__acquire(RCU_BH);
>  	rcu_lock_acquire(&rcu_bh_lock_map);
>  	RCU_LOCKDEP_WARN(!rcu_is_watching(),
>  			 "rcu_read_lock_bh() used illegally while idle");
> +#endif
>  }
>  
>  /*
> @@ -650,10 +652,12 @@ static inline void rcu_read_lock_bh(void)
>   */
>  static inline void rcu_read_unlock_bh(void)
>  {
> +#ifndef CONFIG_PREEMPT_RT_FULL
>  	RCU_LOCKDEP_WARN(!rcu_is_watching(),
>  			 "rcu_read_unlock_bh() used illegally while idle");
>  	rcu_lock_release(&rcu_bh_lock_map);
>  	__release(RCU_BH);
> +#endif
>  	local_bh_enable();
>  }
>  
> diff --git a/kernel/rcu/update.c b/kernel/rcu/update.c
> index 3700b730ea55..eb653a329e0b 100644
> --- a/kernel/rcu/update.c
> +++ b/kernel/rcu/update.c
> @@ -307,7 +307,11 @@ int rcu_read_lock_bh_held(void)
>  		return 0;
>  	if (!rcu_lockdep_current_cpu_online())
>  		return 0;
> +#ifdef CONFIG_PREEMPT_RT_FULL
> +	return lock_is_held(&rcu_lock_map) || irqs_disabled();
> +#else
>  	return in_softirq() || irqs_disabled();
> +#endif
>  }
>  EXPORT_SYMBOL_GPL(rcu_read_lock_bh_held);
>  
> diff --git a/kernel/softirq.c b/kernel/softirq.c
> index 473369122ddd..eb46dd3ff92d 100644
> --- a/kernel/softirq.c
> +++ b/kernel/softirq.c
> @@ -121,8 +121,10 @@ void __local_bh_disable_ip(unsigned long ip, unsigned int cnt)
>  	long soft_cnt;
>  
>  	WARN_ON_ONCE(in_irq());
> -	if (!in_atomic())
> +	if (!in_atomic()) {
>  		local_lock(bh_lock);
> +		rcu_read_lock();
> +	}
>  	soft_cnt = this_cpu_inc_return(softirq_counter);
>  	WARN_ON_ONCE(soft_cnt == 0);
>  
> @@ -155,8 +157,10 @@ void _local_bh_enable(void)
>  	local_irq_restore(flags);
>  #endif
>  
> -	if (!in_atomic())
> +	if (!in_atomic()) {
> +		rcu_read_unlock();
>  		local_unlock(bh_lock);
> +	}
>  }
>  
>  void _local_bh_enable_rt(void)
> @@ -189,8 +193,10 @@ void __local_bh_enable_ip(unsigned long ip, unsigned int cnt)
>  	WARN_ON_ONCE(count < 0);
>  	local_irq_enable();
>  
> -	if (!in_atomic())
> +	if (!in_atomic()) {
> +		rcu_read_unlock();
>  		local_unlock(bh_lock);
> +	}
>  
>  	preempt_check_resched();
>  }

And I have to ask...

What did you do to test this change to kernel/softirq.c?  My past attempts
to do this sort of thing have always run afoul of open-coded BH transitions.

							Thanx, Paul