[程序员] Linux: READ_ONCE

/* * Prevent the compiler from merging or refetching reads or writes. The * compiler is also forbidden from reordering successive instances of * READ_ONCE and WRITE_ONCE, but only when the compiler is aware of some * particular ordering. One way to make the compiler aware of ordering is to * put the two invocations of READ_ONCE or WRITE_ONCE in different C * statements. * * These two macros will also work on aggregate data types like structs or * unions. If the size of the accessed data type exceeds the word size of * the machine (e.g., 32 bits or 64 bits) READ_ONCE() and WRITE_ONCE() will * fall back to memcpy and print a compile-time warning. * * Their two major use cases are: (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. */#define READ_ONCE(x)					\({							\	union { typeof(x) __val; char __c[1]; } __u =	\		{ .__c = { 0 } };			\	__read_once_size(&(x), __u.__c, sizeof(x));	\	__u.__val;					\})