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Subject[PATCH 2/4] barrier.h: Move smp_mb__after_unlock_lock to barrier.h
Date
spin_unlock() + spin_lock() together do not form a full memory barrier:

a=1;
spin_unlock(&b);
spin_lock(&c);
+ smp_mb__after_unlock_lock();
d=1;

Without the smp_mb__after_unlock_lock(), other CPUs can observe the
write to d without seeing the write to a.

Signed-off-by: Manfred Spraul <manfred@colorfullife.com>
---
include/asm-generic/barrier.h | 16 ++++++++++++++++
kernel/rcu/tree.h | 12 ------------
2 files changed, 16 insertions(+), 12 deletions(-)

diff --git a/include/asm-generic/barrier.h b/include/asm-generic/barrier.h
index fe297b5..9b4d28f 100644
--- a/include/asm-generic/barrier.h
+++ b/include/asm-generic/barrier.h
@@ -244,6 +244,22 @@ do { \
smp_acquire__after_ctrl_dep(); \
VAL; \
})
+
+#ifndef smp_mb__after_unlock_lock
+/*
+ * Place this after a lock-acquisition primitive to guarantee that
+ * an UNLOCK+LOCK pair act as a full barrier. This guarantee applies
+ * if the UNLOCK and LOCK are executed by the same CPU or if the
+ * UNLOCK and LOCK operate on the same lock variable.
+ */
+#ifdef CONFIG_PPC
+#define smp_mb__after_unlock_lock() smp_mb() /* Full ordering for lock. */
+#else /* #ifdef CONFIG_PPC */
+#define smp_mb__after_unlock_lock() do { } while (0)
+#endif /* #else #ifdef CONFIG_PPC */
+
+#endif
+
#endif

#endif /* !__ASSEMBLY__ */
diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
index e99a523..a0cd9ab 100644
--- a/kernel/rcu/tree.h
+++ b/kernel/rcu/tree.h
@@ -687,18 +687,6 @@ static inline void rcu_nocb_q_lengths(struct rcu_data *rdp, long *ql, long *qll)
#endif /* #ifdef CONFIG_RCU_TRACE */

/*
- * Place this after a lock-acquisition primitive to guarantee that
- * an UNLOCK+LOCK pair act as a full barrier. This guarantee applies
- * if the UNLOCK and LOCK are executed by the same CPU or if the
- * UNLOCK and LOCK operate on the same lock variable.
- */
-#ifdef CONFIG_PPC
-#define smp_mb__after_unlock_lock() smp_mb() /* Full ordering for lock. */
-#else /* #ifdef CONFIG_PPC */
-#define smp_mb__after_unlock_lock() do { } while (0)
-#endif /* #else #ifdef CONFIG_PPC */
-
-/*
* Wrappers for the rcu_node::lock acquire and release.
*
* Because the rcu_nodes form a tree, the tree traversal locking will observe
--
2.5.5
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