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authorLinus Torvalds <torvalds@linux-foundation.org>2015-02-20 15:46:31 -0800
committerSasha Levin <sasha.levin@oracle.com>2015-06-28 13:39:28 -0400
commitd24b9b8d95f0df8c3b1430ff76396fd705cc0140 (patch)
parentac54513c498163cb043c2564d8062596a788f4a2 (diff)
kernel: make READ_ONCE() valid on const arguments
[ Upstream commit dd36929720f40f17685e841ae0d4c581c165ea60 ] 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@kernel.org> Cc: Christian Borntraeger <borntraeger@de.ibm.com> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org> Signed-off-by: Sasha Levin <sasha.levin@oracle.com>
1 files changed, 3 insertions, 3 deletions
diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index 33063f872ee3..000c5f90f08c 100644
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -198,7 +198,7 @@ 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;
@@ -255,10 +255,10 @@ 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__ */