[PATCH 2/2] add rcu_assign_index() if ever needed

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

Hello again!

This is a speculative patch that as far as I can tell is not yet required.
If anyone applies RCU to a data structure allocated out of an array, using
array indexes in place of pointers to link the array elements together,
then the rcu_assign_index() function in this patch will be needed to
assign a given element's array index to the RCU-traversed index.  The
implementation is exactly that of the old rcu_assign_pointer(), so is
extremely well tested.

The existing rcu_assign_pointer() will emit a compiler warning in cases
where rcu_assign_index() is required.

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

 rcupdate.h |   18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff -urpNa -X dontdiff linux-2.6.24-rap/include/linux/rcupdate.h linux-2.6.24-rai/include/linux/rcupdate.h
--- linux-2.6.24-rap/include/linux/rcupdate.h	2008-02-13 13:36:47.000000000 -0800
+++ linux-2.6.24-rai/include/linux/rcupdate.h	2008-02-13 10:55:40.000000000 -0800
@@ -286,6 +286,24 @@ extern struct lockdep_map rcu_lock_map;
 	})
 
 /**
+ * rcu_assign_index - assign (publicize) a index of a newly
+ * initialized array elementg 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 index assignment.  More importantly, this
+ * call documents which indexes will be dereferenced by RCU read-side
+ * code.
+ */
+
+#define rcu_assign_index(p, v)	({ \
+					smp_wmb(); \
+					(p) = (v); \
+				})
+
+/**
  * synchronize_sched - block until all CPUs have exited any non-preemptive
  * kernel code sequences.
  *