Remove rcu_assign_pointer() penalty for NULL pointers

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@linux.vnet.ibm.com>
Acked-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 7 insertions, 4 deletions

diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index d32c14de270e..37a642c54871 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -174,10 +174,13 @@ struct rcu_head {
  * 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