Re: [PATCH tip/core/rcu 04/13] rcu: Don't disable preemption for Tiny and Tree RCU readers

[Josh Triplett]

On Tue, Oct 06, 2015 at 09:13:39AM -0700, Paul E. McKenney wrote:
> From: Boqun Feng <boqun.feng@xxxxxxxxx>
> 
> Because preempt_disable() maps to barrier() for non-debug builds,
> it forces the compiler to spill and reload registers.  Because Tree
> RCU and Tiny RCU now only appear in CONFIG_PREEMPT=n builds, these
> barrier() instances generate needless extra code for each instance of
> rcu_read_lock() and rcu_read_unlock().  This extra code slows down Tree
> RCU and bloats Tiny RCU.
> 
> This commit therefore removes the preempt_disable() and preempt_enable()
> from the non-preemptible implementations of __rcu_read_lock() and
> __rcu_read_unlock(), respectively.  However, for debug purposes,
> preempt_disable() and preempt_enable() are still invoked if
> CONFIG_PREEMPT_COUNT=y, because this allows detection of sleeping inside
> atomic sections in non-preemptible kernels.
> 
> This is based on an earlier patch by Paul E. McKenney, fixing
> a bug encountered in kernels built with CONFIG_PREEMPT=n and
> CONFIG_PREEMPT_COUNT=y.

This also adds explicit barrier() calls to several internal RCU
functions, but the commit message doesn't explain those at all.

> Signed-off-by: Boqun Feng <boqun.feng@xxxxxxxxx>
> Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
> ---
>  include/linux/rcupdate.h | 6 ++++--
>  include/linux/rcutiny.h  | 1 +
>  kernel/rcu/tree.c        | 9 +++++++++
>  3 files changed, 14 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> index d63bb77dab35..6c3ceceb6148 100644
> --- a/include/linux/rcupdate.h
> +++ b/include/linux/rcupdate.h
> @@ -297,12 +297,14 @@ void synchronize_rcu(void);
>  
>  static inline void __rcu_read_lock(void)
>  {
> -	preempt_disable();
> +	if (IS_ENABLED(CONFIG_PREEMPT_COUNT))
> +		preempt_disable();
>  }
>  
>  static inline void __rcu_read_unlock(void)
>  {
> -	preempt_enable();
> +	if (IS_ENABLED(CONFIG_PREEMPT_COUNT))
> +		preempt_enable();
>  }
>  
>  static inline void synchronize_rcu(void)
> diff --git a/include/linux/rcutiny.h b/include/linux/rcutiny.h
> index c8a0722f77ea..4c1aaf9cce7b 100644
> --- a/include/linux/rcutiny.h
> +++ b/include/linux/rcutiny.h
> @@ -216,6 +216,7 @@ static inline bool rcu_is_watching(void)
>  
>  static inline void rcu_all_qs(void)
>  {
> +	barrier(); /* Avoid RCU read-side critical sections leaking across. */
>  }
>  
>  #endif /* __LINUX_RCUTINY_H */
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index b9d9e0249e2f..93c0f23c3e45 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -337,12 +337,14 @@ static void rcu_momentary_dyntick_idle(void)
>   */
>  void rcu_note_context_switch(void)
>  {
> +	barrier(); /* Avoid RCU read-side critical sections leaking down. */
>  	trace_rcu_utilization(TPS("Start context switch"));
>  	rcu_sched_qs();
>  	rcu_preempt_note_context_switch();
>  	if (unlikely(raw_cpu_read(rcu_sched_qs_mask)))
>  		rcu_momentary_dyntick_idle();
>  	trace_rcu_utilization(TPS("End context switch"));
> +	barrier(); /* Avoid RCU read-side critical sections leaking up. */
>  }
>  EXPORT_SYMBOL_GPL(rcu_note_context_switch);
>  
> @@ -353,12 +355,19 @@ EXPORT_SYMBOL_GPL(rcu_note_context_switch);
>   * RCU flavors in desperate need of a quiescent state, which will normally
>   * be none of them).  Either way, do a lightweight quiescent state for
>   * all RCU flavors.
> + *
> + * The barrier() calls are redundant in the common case when this is
> + * called externally, but just in case this is called from within this
> + * file.
> + *
>   */
>  void rcu_all_qs(void)
>  {
> +	barrier(); /* Avoid RCU read-side critical sections leaking down. */
>  	if (unlikely(raw_cpu_read(rcu_sched_qs_mask)))
>  		rcu_momentary_dyntick_idle();
>  	this_cpu_inc(rcu_qs_ctr);
> +	barrier(); /* Avoid RCU read-side critical sections leaking up. */
>  }
>  EXPORT_SYMBOL_GPL(rcu_all_qs);
>  
> -- 
> 2.5.2
> 
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