[PATCH tip/core/rcu 12/16] srcu: Add API for barrier after srcu_read_unlock()

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

From: "Michael S. Tsirkin" <mst@redhat.com>

The srcu_read_unlock() implementation currently contains a full memory
barrier, but that has not always been the case and it someday again might
not be the case.  However, given that the memory barrier is there, it is
useful to be able to rely on it.  This commit therefore adds a new
smp_mb__after_srcu_read_unlock() API that, together with a preceding
srcu_read_unlock(), is guaranteed to provide a full barrier.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: "Lai Jiangshan" <laijs@cn.fujitsu.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
 include/linux/srcu.h | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/include/linux/srcu.h b/include/linux/srcu.h
index c114614ed172..9b058eecd403 100644
--- a/include/linux/srcu.h
+++ b/include/linux/srcu.h
@@ -237,4 +237,18 @@ static inline void srcu_read_unlock(struct srcu_struct *sp, int idx)
 	__srcu_read_unlock(sp, idx);
 }
 
+/**
+ * smp_mb__after_srcu_read_unlock - ensure full ordering after srcu_read_unlock
+ *
+ * Converts the preceding srcu_read_unlock into a two-way memory barrier.
+ *
+ * Call this after srcu_read_unlock, to guarantee that all memory operations
+ * that occur after smp_mb__after_srcu_read_unlock will appear to happen after
+ * the preceding srcu_read_unlock.
+ */
+static inline void smp_mb__after_srcu_read_unlock(void)
+{
+	/* __srcu_read_unlock has smp_mb() internally so nothing to do here. */
+}
+
 #endif
-- 
1.8.1.5