Merge tag 'libnvdimm-for-4.2' of git://git.kernel.org/pub/scm/linux/kernel/git/djbw...

[firefly-linux-kernel-4.4.55.git] / include / linux / compiler.h

diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index 9a528d94549840126720897cbc98d6a2cdb2e9ae..26fc8bc77f85644adf8cb955579252b1eebcc423 100644 (file)
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -252,7 +252,23 @@ static __always_inline void __write_once_size(volatile void *p, void *res, int s
        ({ union { typeof(x) __val; char __c[1]; } __u; __read_once_size(&(x), __u.__c, sizeof(x)); __u.__val; })
 
 #define WRITE_ONCE(x, val) \
-       ({ typeof(x) __val = (val); __write_once_size(&(x), &__val, sizeof(__val)); __val; })
+       ({ union { typeof(x) __val; char __c[1]; } __u = { .__val = (val) }; __write_once_size(&(x), __u.__c, sizeof(x)); __u.__val; })
+
+/**
+ * READ_ONCE_CTRL - Read a value heading a control dependency
+ * @x: The value to be read, heading the control dependency
+ *
+ * Control dependencies are tricky.  See Documentation/memory-barriers.txt
+ * for important information on how to use them.  Note that in many cases,
+ * use of smp_load_acquire() will be much simpler.  Control dependencies
+ * should be avoided except on the hottest of hotpaths.
+ */
+#define READ_ONCE_CTRL(x) \
+({ \
+       typeof(x) __val = READ_ONCE(x); \
+       smp_read_barrier_depends(); /* Enforce control dependency. */ \
+       __val; \
+})
 
 #endif /* __KERNEL__ */
 
@@ -452,7 +468,7 @@ static __always_inline void __write_once_size(volatile void *p, void *res, int s
  * with an explicit memory barrier or atomic instruction that provides the
  * required ordering.
  *
- * If possible use READ_ONCE/ASSIGN_ONCE instead.
+ * If possible use READ_ONCE()/WRITE_ONCE() instead.
  */
 #define __ACCESS_ONCE(x) ({ \
         __maybe_unused typeof(x) __var = (__force typeof(x)) 0; \