[Devel] [PATCH rh7 3/8] kernel: make READ_ONCE() valid on const arguments

[Andrey Ryabinin]

From: Linus Torvalds <torvalds at linux-foundation.org>

The use of READ_ONCE() causes lots of warnings witht he pending paravirt
spinlock fixes, because those ends up having passing a member to a
'const' structure to READ_ONCE().

There should certainly be nothing wrong with using READ_ONCE() with a
const source, but the helper function __read_once_size() would cause
warnings because it would drop the 'const' qualifier, but also because
the destination would be marked 'const' too due to the use of 'typeof'.

Use a union of types in READ_ONCE() to avoid this issue.

Also make sure to use parenthesis around the macro arguments to avoid
possible operator precedence issues.

Tested-by: Ingo Molnar <mingo at kernel.org>
Cc: Christian Borntraeger <borntraeger at de.ibm.com>
Signed-off-by: Linus Torvalds <torvalds at linux-foundation.org>
(cherry picked from commit dd36929720f40f17685e841ae0d4c581c165ea60)
Signed-off-by: Andrey Ryabinin <aryabinin at virtuozzo.com>
---
 include/linux/compiler.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index 0737107..39fe1dc 100644
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -187,7 +187,7 @@ static __always_inline void data_access_exceeds_word_size(void)
 {
 }
 
-static __always_inline void __read_once_size(volatile void *p, void *res, int size)
+static __always_inline void __read_once_size(const volatile void *p, void *res, int size)
 {
 	switch (size) {
 	case 1: *(__u8 *)res = *(volatile __u8 *)p; break;
@@ -244,7 +244,7 @@ static __always_inline void __assign_once_size(volatile void *p, void *res, int
  */
 
 #define READ_ONCE(x) \
-	({ typeof(x) __val; __read_once_size(&x, &__val, sizeof(__val)); __val; })
+	({ union { typeof(x) __val; char __c[1]; } __u; __read_once_size(&(x), __u.__c, sizeof(x)); __u.__val; })
 
 #define ASSIGN_ONCE(val, x) \
 	({ typeof(x) __val; __val = val; __assign_once_size(&x, &__val, sizeof(__val)); __val; })
-- 
2.4.10