linux-kernel - [PATCH 2/4] barrier.h: Move smp_mb__after_unlock

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Date:   Sun, 28 Aug 2016 18:31:15 +0200
From:   Manfred Spraul <manfred@...orfullife.com>
To:     benh@...nel.crashing.org, paulmck@...ux.vnet.ibm.com,
        Ingo Molnar <mingo@...e.hu>, Boqun Feng <boqun.feng@...il.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     LKML <linux-kernel@...r.kernel.org>, 1vier1@....de,
        Davidlohr Bueso <dave@...olabs.net>,
        Manfred Spraul <manfred@...orfullife.com>
Subject: [PATCH 2/4] barrier.h: Move smp_mb__after_unlock_lock to barrier.h

spin_unlock() + spin_lock() together do not form a full memory barrier:
(everything initialized to 0)

CPU1:
  a=1;
  spin_unlock(&b);
  spin_lock(&c);
+ smp_mb__after_unlock_lock();
  r1=d;

CPU2:
  d=1;
  smp_mb();
  r2=a;

Without the smp_mb__after_unlock_lock(), r1==0 && r2==0 would
be possible.

Signed-off-by: Manfred Spraul <manfred@...orfullife.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