Re: [Qemu-devel] [PATCH] rcu: optimize rcu_read_lock

[Wen Congyang <wency@cn.fujitsu.com>]

On 12/16/2015 07:32 PM, Paolo Bonzini wrote:
> rcu_read_lock cannot change rcu_gp_ongoing from true to false
> (the previous value of p_rcu_reader->ctr is zero), hence
> there is no need to check p_rcu_reader->waiting and wake up
> a concurrent synchronize_rcu.
> 
> While at it mark the wakeup as unlikely in rcu_read_unlock.
> 
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

Reviewed-by: Wen Congyang <wency@cn.fujitsu.com>

> ---
>  include/qemu/rcu.h | 6 +-----
>  1 file changed, 1 insertion(+), 5 deletions(-)
> 
> diff --git a/include/qemu/rcu.h b/include/qemu/rcu.h
> index f6d1d56..7c7cca7 100644
> --- a/include/qemu/rcu.h
> +++ b/include/qemu/rcu.h
> @@ -88,10 +88,6 @@ static inline void rcu_read_lock(void)
>  
>      ctr = atomic_read(&rcu_gp_ctr);
>      atomic_xchg(&p_rcu_reader->ctr, ctr);
> -    if (atomic_read(&p_rcu_reader->waiting)) {
> -        atomic_set(&p_rcu_reader->waiting, false);
> -        qemu_event_set(&rcu_gp_event);
> -    }
>  }
>  
>  static inline void rcu_read_unlock(void)
> @@ -104,7 +100,7 @@ static inline void rcu_read_unlock(void)
>      }
>  
>      atomic_xchg(&p_rcu_reader->ctr, 0);
> -    if (atomic_read(&p_rcu_reader->waiting)) {
> +    if (unlikely(atomic_read(&p_rcu_reader->waiting))) {
>          atomic_set(&p_rcu_reader->waiting, false);
>          qemu_event_set(&rcu_gp_event);
>      }
>