On Fri, Sep 11, 2015 at 10:19:47AM +0800, Boqun Feng wrote:
> Subject: [PATCH 01/27] rcu: Don't disable preemption for Tiny and Tree RCU
>  readers
> 
> 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.
> 
> For debug purposes, preempt_disable() and preempt_enable() are still
> kept if CONFIG_PREEMPT_COUNT=y, which makes the detection of sleeping
> inside atomic sections still work in non-preemptible kernels.
> 
> Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> ---
>  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 d63bb77..6c3cece 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();

preempt_disable() is a no-op when !CONFIG_PREEMPT_COUNT, right?
Or rather it's a barrier(), which is anyway implied by rcu_read_lock().

So perhaps we can get rid of the IS_ENABLED() check?