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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>
This commit is contained in:
Paolo Bonzini 2015-12-16 12:32:22 +01:00
parent 3cc8f88499
commit f6d153f1bf
1 changed files with 1 additions and 5 deletions
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6
include/qemu/rcu.h
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@ -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);
}