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[PULL,40/45] rcu: optimize rcu_read_lock

Message ID 1450374401-31352-41-git-send-email-pbonzini@redhat.com
State New
Headers show

Commit Message

Paolo Bonzini Dec. 17, 2015, 5:46 p.m. UTC 
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 | 6 +-----
 1 file changed, 1 insertion(+), 5 deletions(-)
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Patch

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);
     }