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Subject[RFC PATCH]rcu,alpha: avoid smp_read_barrier_depends in rcu_dereference when pointer==NULL
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); \
})





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