kernel: make READ_ONCE() valid on const arguments (dd369297) · Commits · 戴 / test

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>

include/linux/compiler.h

+3 −3

Original line number	Diff line number	Diff line
		@@ -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__ */

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