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Subject[PATCH 1/3] 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>

---
rcupdate.h | 18 ++++++++++++++++++
1 files changed, 18 insertions(+)
diff -urpN -X ../dontdiff linux-2.5/include/linux/rcupdate.h linux-2.5-rap/include/linux/rcupdate.h
--- linux-2.5/include/linux/rcupdate.h Tue Sep 7 10:04:29 2004
+++ linux-2.5-rap/include/linux/rcupdate.h Tue Sep 7 12:12:09 2004
@@ -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);
-
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