linux-kernel - [PATCH 3.19 173/175] kernel: make READ

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Date:	Tue,  3 Mar 2015 22:15:51 -0800
From:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:	linux-kernel@...r.kernel.org
Cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	stable@...r.kernel.org, Ingo Molnar <mingo@...nel.org>,
	Christian Borntraeger <borntraeger@...ibm.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: [PATCH 3.19 173/175] kernel: make READ_ONCE() valid on const arguments

3.19-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Linus Torvalds <torvalds@...ux-foundation.org>

commit dd36929720f40f17685e841ae0d4c581c165ea60 upstream.

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@...nel.org>
Cc: Christian Borntraeger <borntraeger@...ibm.com>
Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

---
 include/linux/compiler.h |    6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -198,7 +198,7 @@ static __always_inline void data_access_
 {
 }
 
-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;
@@ -255,10 +255,10 @@ static __always_inline void __write_once
  */
 
 #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 WRITE_ONCE(x, val) \
-	({ typeof(x) __val; __val = val; __write_once_size(&x, &__val, sizeof(__val)); __val; })
+	({ typeof(x) __val = (val); __write_once_size(&(x), &__val, sizeof(__val)); __val; })
 
 #endif /* __KERNEL__ */
 


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