rcu: create new rcu_access_index() and use in mce

The MCE subsystem needs to sample an RCU-protected index outside of
any protection for that index.  If this was a pointer, we would use
rcu_access_pointer(), but there is no corresponding rcu_access_index().
This commit therefore creates an rcu_access_index() and applies it
to MCE.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Tested-by: Zdenek Kabelac <zkabelac@redhat.com>

1 files changed, 20 insertions, 0 deletions

diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index af5614856285..ff422d2b7f90 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -339,6 +339,12 @@ extern int rcu_my_thread_group_empty(void);
                 ((typeof(*p) __force __kernel *)(p)); \
         })
 
+#define __rcu_access_index(p, space) \
+        ({ \
+                typeof(p) _________p1 = ACCESS_ONCE(p); \
+                rcu_dereference_sparse(p, space); \
+                (_________p1); \
+        })
 #define __rcu_dereference_index_check(p, c) \
         ({ \
                 typeof(p) _________p1 = ACCESS_ONCE(p); \
@@ -429,6 +435,20 @@ extern int rcu_my_thread_group_empty(void);
 #define rcu_dereference_raw(p) rcu_dereference_check(p, 1) /*@@@ needed? @@@*/
 
 /**
+ * rcu_access_index() - fetch RCU index with no dereferencing
+ * @p: The index to read
+ *
+ * Return the value of the specified RCU-protected index, but omit the
+ * smp_read_barrier_depends() and keep the ACCESS_ONCE().  This is useful
+ * when the value of this index is accessed, but the index is not
+ * dereferenced, for example, when testing an RCU-protected index against
+ * -1.  Although rcu_access_index() may also be used in cases where
+ * update-side locks prevent the value of the index from changing, you
+ * should instead use rcu_dereference_index_protected() for this use case.
+ */
+#define rcu_access_index(p) __rcu_access_index((p), __rcu)
+
+/**
  * rcu_dereference_index_check() - rcu_dereference for indices with debug checking
  * @p: The pointer to read, prior to dereferencing
  * @c: The conditions under which the dereference will take place