Commit dd369297 authored by Linus Torvalds's avatar Linus Torvalds
Browse files

kernel: make READ_ONCE() valid on const arguments

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: default avatarIngo Molnar <>
Cc: Christian Borntraeger <>
Signed-off-by: default avatarLinus Torvalds <>
parent 4fbd0a81
...@@ -202,7 +202,7 @@ static __always_inline void data_access_exceeds_word_size(void) ...@@ -202,7 +202,7 @@ 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) { switch (size) {
case 1: *(__u8 *)res = *(volatile __u8 *)p; break; case 1: *(__u8 *)res = *(volatile __u8 *)p; break;
...@@ -259,10 +259,10 @@ static __always_inline void __write_once_size(volatile void *p, void *res, int s ...@@ -259,10 +259,10 @@ static __always_inline void __write_once_size(volatile void *p, void *res, int s
*/ */
#define READ_ONCE(x) \ #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) \ #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__ */ #endif /* __KERNEL__ */
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