kernel: tighten rules for ACCESS ONCE

 * to make the compiler aware of ordering is to put the two invocations of

 * ACCESS_ONCE() in different C statements.

 *

 * This macro does absolutely -nothing- to prevent the CPU from reordering,

 * merging, or refetching absolutely anything at any time.  Its main intended

 * use is to mediate communication between process-level code and irq/NMI

 * handlers, all running on the same CPU.

 * ACCESS_ONCE will only work on scalar types. For union types, ACCESS_ONCE

 * on a union member will work as long as the size of the member matches the

 * size of the union and the size is smaller than word size.

 *

 * The major use cases of ACCESS_ONCE used to be (1) Mediating communication

 * between process-level code and irq/NMI handlers, all running on the same CPU,

 * and (2) Ensuring that the compiler does not  fold, spindle, or otherwise

 * mutilate accesses that either do not require ordering or that interact

 * with an explicit memory barrier or atomic instruction that provides the

 * required ordering.

 *

 * If possible use READ_ONCE/ASSIGN_ONCE instead.

 */

#define ACCESS_ONCE(x) (*(volatile typeof(x) *)&(x))

#define __ACCESS_ONCE(x) ({ \

	 __maybe_unused typeof(x) __var = 0; \

	(volatile typeof(x) *)&(x); })

#define ACCESS_ONCE(x) (*__ACCESS_ONCE(x))

/* Ignore/forbid kprobes attach on very low level functions marked by this attribute: */

#ifdef CONFIG_KPROBES