rcu: optimize rcu_read_lock

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.

Reviewed-by: Wen Congyang <wency@cn.fujitsu.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <1450265542-4323-1-git-send-email-pbonzini@redhat.com>

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);
     }