哪位好心人解释一下mb() 函数的意思吧？

我在/linux/include/asm-x86/system.h中看到这段代码：

291
292 /*
293  * Force strict CPU ordering.
294  * And yes, this is required on UP too when we're talking
295  * to devices.
296  */
297 #ifdef CONFIG_X86_32
311 #define mb() alternative("lock; addl $0,0(%%esp)", "mfence", X86_FEATURE_XMM2)
312 #define rmb() alternative("lock; addl $0,0(%%esp)", "lfence", X86_FEATURE_XMM2)
313 #define wmb() alternative("lock; addl $0,0(%%esp)", "sfence", X86_FEATURE_XMM)
314 #else
315 #define mb()    asm volatile("mfence":::"memory")
316 #define rmb()   asm volatile("lfence":::"memory")
317 #define wmb()   asm volatile("sfence" ::: "memory")
318 #endif
319


我想请问，alternative是不是说如果没有mfence指令就用lock; addl $0,0(%%esp)来代替？

memory barrier?

mb内存栅，作为一种数据同步的手段