From: Linus Torvalds Date: Fri, 20 Feb 2015 23:46:31 +0000 (-0800) Subject: kernel: make READ_ONCE() valid on const arguments X-Git-Tag: v4.0-rc1~28 X-Git-Url: http://git.samba.org/samba.git/?p=sfrench%2Fcifs-2.6.git;a=commitdiff_plain;h=dd36929720f40f17685e841ae0d4c581c165ea60 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: Ingo Molnar Cc: Christian Borntraeger Signed-off-by: Linus Torvalds --- diff --git a/include/linux/compiler.h b/include/linux/compiler.h index d1ec10a940ff..1b45e4a0519b 100644 --- a/include/linux/compiler.h +++ b/include/linux/compiler.h @@ -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) { 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 */ #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__ */