netdev - [PATCH] Remove rcu_assign_pointer() penalty for NULL pointers

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Message-ID: <20071201003721.GA22726@linux.vnet.ibm.com>
Date:	Fri, 30 Nov 2007 16:37:21 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	linux-kernel@...r.kernel.org
Cc:	herbert@...dor.apana.org.au, ego@...ibm.com,
	netdev@...r.kernel.org, bridge@...ts.osdl.org, devel@...nvz.org,
	shemminger@...ux-foundation.org, xemul@...nvz.org,
	dipankar@...ibm.com, akpm@...ux-foundation.org
Subject: [PATCH] Remove rcu_assign_pointer() penalty for NULL pointers

Hello!

The rcu_assign_pointer() primitive currently unconditionally executes
a memory barrier, even when a NULL pointer is being assigned.  This
has lead some to avoid using rcu_assign_pointer() for NULL pointers,
which loses the self-documenting advantages of rcu_assign_pointer()
This patch uses __builtin_const_p() to omit needless memory barriers
for NULL-pointer assignments at compile time with no runtime penalty,
as discussed in the following thread:

	http://www.mail-archive.com/netdev@vger.kernel.org/msg54852.html

Tested on x86_64 and ppc64, also compiled the four cases (NULL/non-NULL
and const/non-const) with gcc version 4.1.2, and hand-checked the
assembly output.

Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
---

 rcupdate.h |   11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff -urpNa -X dontdiff linux-2.6.24-rc1-ego/include/linux/rcupdate.h linux-2.6.24-rc1-egoxu/include/linux/rcupdate.h
--- linux-2.6.24-rc1-ego/include/linux/rcupdate.h	2007-11-06 15:30:02.000000000 -0800
+++ linux-2.6.24-rc1-egoxu/include/linux/rcupdate.h	2007-11-30 09:06:11.000000000 -0800
@@ -191,10 +191,13 @@ static inline void rcu_preempt_boost(voi
  * code.
  */
 
-#define rcu_assign_pointer(p, v)	({ \
-						smp_wmb(); \
-						(p) = (v); \
-					})
+#define rcu_assign_pointer(p, v) \
+	({ \
+		if (!__builtin_constant_p(v) || \
+		    ((v) != NULL)) \
+			smp_wmb(); \
+		(p) = (v); \
+	})
 
 /**
  * synchronize_sched - block until all CPUs have exited any non-preemptive
-
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