/**
 * INIT_LIST_HEAD - Initialize a list_head structure
 * @list: list_head structure to be initialized.
 *
 * Initializes the list_head to point to itself.  If it is a list header,
 * the result is an empty list.
 */
static inline void INIT_LIST_HEAD(struct list_head *list)
{
	WRITE_ONCE(list->next, list);
	WRITE_ONCE(list->prev, list);
}

struct list_head {
	struct list_head *next, *prev;
};

/*
 * Yes, this permits 64-bit accesses on 32-bit architectures. These will
 * actually be atomic in some cases (namely Armv7 + LPAE), but for others we
 * rely on the access being split into 2x32-bit accesses for a 32-bit quantity
 * (e.g. a virtual address) and a strong prevailing wind.
 */
#define compiletime_assert_rwonce_type(t)					\
	compiletime_assert(__native_word(t) || sizeof(t) == sizeof(long long),	\
		"Unsupported access size for {READ,WRITE}_ONCE().")

...

#define __WRITE_ONCE(x, val)						\
do {									\
	*(volatile typeof(x) *)&(x) = (val);				\
} while (0)

...

#define WRITE_ONCE(x, val)						\
do {									\
	compiletime_assert_rwonce_type(x);				\
	__WRITE_ONCE(x, val);						\
} while (0)

/* Is this type a native word size -- useful for atomic operations */
#define __native_word(t) \
	(sizeof(t) == sizeof(char) || sizeof(t) == sizeof(short) || \
	 sizeof(t) == sizeof(int) || sizeof(t) == sizeof(long))

#ifdef __OPTIMIZE__
# define __compiletime_assert(condition, msg, prefix, suffix)		\
	do {								\
		/*							\
		 * __noreturn is needed to give the compiler enough	\
		 * information to avoid certain possibly-uninitialized	\
		 * warnings (regardless of the build failing).		\
		 */							\
		__noreturn extern void prefix ## suffix(void)		\
			__compiletime_error(msg);			\
		if (!(condition))					\
			prefix ## suffix();				\
	} while (0)
#else
# define __compiletime_assert(condition, msg, prefix, suffix) do { } while (0)
#endif

#define _compiletime_assert(condition, msg, prefix, suffix) \
	__compiletime_assert(condition, msg, prefix, suffix)

/**
 * compiletime_assert - break build and emit msg if condition is false
 * @condition: a compile-time constant condition to check
 * @msg:       a message to emit if condition is false
 *
 * In tradition of POSIX assert, this macro will break the build if the
 * supplied condition is *false*, emitting the supplied error message if the
 * compiler has support to do so.
 */
#define compiletime_assert(condition, msg) \
	_compiletime_assert(condition, msg, __compiletime_assert_, __COUNTER__)

/*
 * Optional: only supported since clang >= 14.0
 *
 *   gcc: https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#index-error-function-attribute
 */
#if __has_attribute(__error__)
# define __compiletime_error(msg)       __attribute__((__error__(msg)))
#else
# define __compiletime_error(msg)
#endif

...

/*
 *   gcc: https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#index-noreturn-function-attribute
 * clang: https://clang.llvm.org/docs/AttributeReference.html#noreturn
 * clang: https://clang.llvm.org/docs/AttributeReference.html#id1
 */
#define __noreturn                      __attribute__((__noreturn__))