[PATCH] RCU: rcu_assign_pointer() removal of memory barriers

This patch adds the rcu_assign_pointer() API that helps reduce the need for
explicit memory barriers in code that uses RCU.  This API buries the required
memory barriers in a macro that also does the assignment.  This has been
tested successfully on i386 and ppc64.

Signed-off-by: <paulmck@us.ibm.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

include/linux/rcupdate.h

@@ -238,6 +238,24 @@ static inline int rcu_pending(int cpu)
 				(_________p1); \
 				})
 
+/**
+ * rcu_assign_pointer - assign (publicize) a pointer to a newly
+ * initialized structure that will be dereferenced by RCU read-side
+ * critical sections.  Returns the value assigned.
+ *
+ * Inserts memory barriers on architectures that require them
+ * (pretty much all of them other than x86), and also prevents
+ * the compiler from reordering the code that initializes the
+ * structure after the pointer assignment.  More importantly, this
+ * call documents which pointers will be dereferenced by RCU read-side
+ * code.
+ */
+
+#define rcu_assign_pointer(p, v)	({ \
+						smp_wmb(); \
+						(p) = (v); \
+					})
+
 extern void rcu_init(void);
 extern void rcu_check_callbacks(int cpu, int user);
 extern void rcu_restart_cpu(int cpu);