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/[EMAIL PROTECTED]/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 <[EMAIL PROTECTED]>
---

 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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