[PATCHv2 09/10] tighten rules for ACCESS ONCE

[Christian Borntraeger <borntraeger@de.ibm.com>]

Now that all non-scalar users of ACCESS_ONCE have been converted
to READ_ONCE or ASSIGN once, lets tighten ACCESS_ONCE to only
work on scalar types.
This variant was proposed by Alexei Starovoitov.

Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
---
 include/linux/compiler.h | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index 0ff01f2..7265a6c 100644
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -411,8 +411,16 @@ static __always_inline void __read_once_size(volatile void *p, void *res, int si
  * merging, or refetching absolutely anything at any time.  Its main intended
  * use is to mediate communication between process-level code and irq/NMI
  * handlers, all running on the same CPU.
+ *
+ * ACCESS_ONCE will only work on scalar types. For union types, ACCESS_ONCE
+ * on a union member will work as long as the size of the member matches the
+ * size of the union and the size is smaller than word size.
+ * If possible READ_ONCE/ASSIGN_ONCE should be used instead.
  */
-#define ACCESS_ONCE(x) (*(volatile typeof(x) *)&(x))
+#define __ACCESS_ONCE(x) ({ \
+	 __maybe_unused typeof(x) __var = 0; \
+	(volatile typeof(x) *)&(x); })
+#define ACCESS_ONCE(x) (*__ACCESS_ONCE(x))
 
 /* Ignore/forbid kprobes attach on very low level functions marked by this attribute: */
 #ifdef CONFIG_KPROBES
-- 
1.9.3