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 <[email protected]> Signed-off-by: Paul E. McKenney <[email protected]> Signed-off-by: Al Viro <[email protected]>
cluon · Oct 28, 2014 · 54ef6df · 54ef6df
1 parent 5fecf3a
commit 54ef6df
Showing 1 changed file with 15 additions and 0 deletions.
diff --git 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