From mboxrd@z Thu Jan  1 00:00:00 1970
From: Lai Jiangshan <laijs@cn.fujitsu.com>
Subject: [RFC PATCH]rcu,alpha: avoid smp_read_barrier_depends in rcu_dereference
 when pointer==NULL
Date: Sat, 21 Jun 2008 17:54:01 +0800
Message-ID: <485CCFB9.9050208@cn.fujitsu.com>
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To: Andrew Morton <akpm@linux-foundation.org>, "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>, linux-arch@vger.kernel.org
Cc: Linux Kernel Mailing List <linux-kernel@vger.kernel.org>

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); \
 				})