[RFC PATCH]rcu,alpha: avoid smp_read_barrier_depends in rcu_dereference when pointer==NULL

[Lai Jiangshan <laijs@cn.fujitsu.com>]

Hi, Paul and Alpha hackers

memory barrier is expensive, I think it's worth that adding an if-statement
to avoid it when pointer == NULL.

This patch change nothing in UP or in other architectures for compiler's
optimization.

			Thanks, Lai Jiangshan


Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
---
diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index d42dbec..8979f29 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -143,7 +143,8 @@ struct rcu_head {
 
 #define rcu_dereference(p)     ({ \
 				typeof(p) _________p1 = ACCESS_ONCE(p); \
-				smp_read_barrier_depends(); \
+				if (_________p1) \
+					smp_read_barrier_depends(); \
 				(_________p1); \
 				})