"All programmers are blind, especially me."
You are right, Jarek. I ran this through gcc, and the following
comes close:
#define rcu_assign_pointer(p, v) \
({ \
if (!__builtin_constant_p(v) || (v)) \
smp_wmb(); \
(p) = (v); \
})
But I am concerned about the following case:
rcu_assign_pointer(global_index, 0);
. . .
x = global_array[rcu_dereference(global_index)];
Since arrays have a zero-th element, we would really want a memory
barrier in this case.
So how about leaving the index-unfriendly version of rcu_assign_pointer()
and adding an rcu_assign_index() as follows?
Thanx, Paul
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/include/linux/rcupdate.h linux-2.6.24-rai/include/linux/rcupdate.h
--- linux-2.6.24/include/linux/rcupdate.h 2008-01-24 14:58:37.000000000 -0800
+++ linux-2.6.24-rai/include/linux/rcupdate.h 2008-02-12 08:04:59.000000000 -0800
@@ -278,6 +278,24 @@ 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.
*
--
