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

[Wen Congyang]

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 

Reviewed-by: Wen Congyang 

> ---
>  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(_gp_ctr);
>  atomic_xchg(_rcu_reader->ctr, ctr);
> -if (atomic_read(_rcu_reader->waiting)) {
> -atomic_set(_rcu_reader->waiting, false);
> -qemu_event_set(_gp_event);
> -}
>  }
>  
>  static inline void rcu_read_unlock(void)
> @@ -104,7 +100,7 @@ static inline void rcu_read_unlock(void)
>  }
>  
>  atomic_xchg(_rcu_reader->ctr, 0);
> -if (atomic_read(_rcu_reader->waiting)) {
> +if (unlikely(atomic_read(_rcu_reader->waiting))) {
>  atomic_set(_rcu_reader->waiting, false);
>  qemu_event_set(_gp_event);
>  }
> 

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

[Paolo Bonzini]

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 
---
 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(_gp_ctr);
 atomic_xchg(_rcu_reader->ctr, ctr);
-if (atomic_read(_rcu_reader->waiting)) {
-atomic_set(_rcu_reader->waiting, false);
-qemu_event_set(_gp_event);
-}
 }
 
 static inline void rcu_read_unlock(void)
@@ -104,7 +100,7 @@ static inline void rcu_read_unlock(void)
 }
 
 atomic_xchg(_rcu_reader->ctr, 0);
-if (atomic_read(_rcu_reader->waiting)) {
+if (unlikely(atomic_read(_rcu_reader->waiting))) {
 atomic_set(_rcu_reader->waiting, false);
 qemu_event_set(_gp_event);
 }
-- 
2.5.0