asm volatile("dmb osh" : : : "memory")是什么？
arm架构有三种内存屏障，分别是：
Data Memory Barrier, Data Synchronization Barrier, and Instruction Synchronization Barrier.
OSH表示DMB 操作仅限于外部共享域（outer shareable domain）。
https://developer.arm.com/documentation/dui0588/a/arm-and-thumb-instructions/miscellaneous-instructions/dmb--dsb--and-isb#:~:text=Data%20Memory%20Barrier%20acts%20as,to%20the%20outer%20shareable%20domain.