Re: [PATCH RT v3 1/5] rcu: Acquire RCU lock when disabling BHs

[Joel Fernandes]

On Wed, Sep 11, 2019 at 05:57:25PM +0100, 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@xxxxxxxxxx>

Reviewed-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>

thanks,

 - Joel

> ---
> v3: Remove change to rcu_read_lock_bh_held(), and move debug portions
> of rcu_read_[un]lock_bh() to separate functions
> ---
>  include/linux/rcupdate.h | 40 ++++++++++++++++++++++++++++++++--------
>  kernel/softirq.c         | 12 +++++++++---
>  2 files changed, 41 insertions(+), 11 deletions(-)
> 
> diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> index 388ace315f32..9ce7c5006a5e 100644
> --- a/include/linux/rcupdate.h
> +++ b/include/linux/rcupdate.h
> @@ -600,6 +600,36 @@ static inline void rcu_read_unlock(void)
>  	rcu_lock_release(&rcu_lock_map); /* Keep acq info for rls diags. */
>  }
>  
> +#ifdef CONFIG_PREEMPT_RT_FULL
> +/*
> + * On RT, local_bh_disable() calls rcu_read_lock() -- no need to
> + * track it twice.
> + */
> +static inline void rcu_bh_lock_acquire(void)
> +{
> +}
> +
> +static inline void rcu_bh_lock_release(void)
> +{
> +}
> +#else
> +static inline void rcu_bh_lock_acquire(void)
> +{
> +	__acquire(RCU_BH);
> +	rcu_lock_acquire(&rcu_bh_lock_map);
> +	RCU_LOCKDEP_WARN(!rcu_is_watching(),
> +			 "rcu_read_lock_bh() used illegally while idle");
> +}
> +
> +static inline void rcu_bh_lock_release(void)
> +{
> +	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
> +
>  /**
>   * rcu_read_lock_bh() - mark the beginning of an RCU-bh critical section
>   *
> @@ -615,10 +645,7 @@ static inline void rcu_read_unlock(void)
>  static inline void rcu_read_lock_bh(void)
>  {
>  	local_bh_disable();
> -	__acquire(RCU_BH);
> -	rcu_lock_acquire(&rcu_bh_lock_map);
> -	RCU_LOCKDEP_WARN(!rcu_is_watching(),
> -			 "rcu_read_lock_bh() used illegally while idle");
> +	rcu_bh_lock_acquire();
>  }
>  
>  /*
> @@ -628,10 +655,7 @@ static inline void rcu_read_lock_bh(void)
>   */
>  static inline void rcu_read_unlock_bh(void)
>  {
> -	RCU_LOCKDEP_WARN(!rcu_is_watching(),
> -			 "rcu_read_unlock_bh() used illegally while idle");
> -	rcu_lock_release(&rcu_bh_lock_map);
> -	__release(RCU_BH);
> +	rcu_bh_lock_release();
>  	local_bh_enable();
>  }
>  
> diff --git a/kernel/softirq.c b/kernel/softirq.c
> index d16d080a74f7..6080c9328df1 100644
> --- a/kernel/softirq.c
> +++ b/kernel/softirq.c
> @@ -115,8 +115,10 @@ 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);
>  	current->softirq_count += SOFTIRQ_DISABLE_OFFSET;
> @@ -151,8 +153,10 @@ void _local_bh_enable(void)
>  #endif
>  
>  	current->softirq_count -= SOFTIRQ_DISABLE_OFFSET;
> -	if (!in_atomic())
> +	if (!in_atomic()) {
> +		rcu_read_unlock();
>  		local_unlock(bh_lock);
> +	}
>  }
>  
>  void _local_bh_enable_rt(void)
> @@ -185,8 +189,10 @@ 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);
> +	}
>  
>  	current->softirq_count -= SOFTIRQ_DISABLE_OFFSET;
>  	preempt_check_resched();
> -- 
> 1.8.3.1
>