Commit f6d153f1 authored by Paolo Bonzini's avatar Paolo Bonzini
Browse files

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: default avatarWen Congyang <wency@cn.fujitsu.com>
Signed-off-by: default avatarPaolo Bonzini <pbonzini@redhat.com>
Message-Id: <1450265542-4323-1-git-send-email-pbonzini@redhat.com>
parent 3cc8f884

include/qemu/rcu.h

+1 −5
Original line number Diff line number Diff line
@@ -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);

    }