rcu: Provide counterpart to rcu_dereference() for non-RCU situations

Although rcu_dereference() and friends can be used in situations where
object lifetimes are being managed by something other than RCU, the
resulting sparse and lockdep-RCU noise can be annoying.  This commit
therefore supplies a lockless_dereference(), which provides the
protection for dereferences without the RCU-related debugging noise.

Reported-by: Al Viro <viro@ZenIV.linux.org.uk>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index a4a819f..53ff1a7 100644 (file)
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -616,6 +616,21 @@ static inline void rcu_preempt_sleep_check(void)
  */
 #define RCU_INITIALIZER(v) (typeof(*(v)) __force __rcu *)(v)
 
+/**
+ * lockless_dereference() - safely load a pointer for later dereference
+ * @p: The pointer to load
+ *
+ * Similar to rcu_dereference(), but for situations where the pointed-to
+ * object's lifetime is managed by something other than RCU.  That
+ * "something other" might be reference counting or simple immortality.
+ */
+#define lockless_dereference(p) \
+({ \
+       typeof(p) _________p1 = ACCESS_ONCE(p); \
+       smp_read_barrier_depends(); /* Dependency order vs. p above. */ \
+       (_________p1); \
+})
+
 /**
  * rcu_assign_pointer() - assign to RCU-protected pointer
  * @p: pointer to assign to