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 <mingo@kernel.org> Cc: Christian Borntraeger <borntraeger@de.ibm.com> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
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@ -202,7 +202,7 @@ static __always_inline void data_access_exceeds_word_size(void)
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{
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}
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static __always_inline void __read_once_size(volatile void *p, void *res, int size)
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static __always_inline void __read_once_size(const volatile void *p, void *res, int size)
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{
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switch (size) {
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case 1: *(__u8 *)res = *(volatile __u8 *)p; break;
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@ -259,10 +259,10 @@ static __always_inline void __write_once_size(volatile void *p, void *res, int s
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*/
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#define READ_ONCE(x) \
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({ typeof(x) __val; __read_once_size(&x, &__val, sizeof(__val)); __val; })
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({ union { typeof(x) __val; char __c[1]; } __u; __read_once_size(&(x), __u.__c, sizeof(x)); __u.__val; })
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#define WRITE_ONCE(x, val) \
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({ typeof(x) __val; __val = val; __write_once_size(&x, &__val, sizeof(__val)); __val; })
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({ typeof(x) __val = (val); __write_once_size(&(x), &__val, sizeof(__val)); __val; })
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#endif /* __KERNEL__ */
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