X86 平台 原子操作的LOCK_PREFIX前缀提供了内存屏障

 基于2.6.32

/**
 * clear_bit - Clears a bit in memory
 * @nr: Bit to clear
 * @addr: Address to start counting from
 *
 * clear_bit() is atomic and may not be reordered.  However, it does
 * not contain a memory barrier, so if it is used for locking purposes,
 * you should call smp_mb__before_clear_bit() and/or smp_mb__after_clear_bit()
 * in order to ensure changes are visible on other processors.
 */

    static __always_inline void  clear_bit(int nr, volatile unsigned long *addr)
    {
        if (IS_IMMEDIATE(nr)) {
            asm volatile(LOCK_PREFIX "andb %1,%0"
                        : CONST_MASK_ADDR(nr, addr)
                        : "iq" ((u8)~CONST_MASK(nr)));
        } else {
            asm volatile(LOCK_PREFIX "btr %1,%0"
            : BITOP_ADDR(addr)
            : "Ir" (nr));
        }
    }
    提供了barrier的含义是优化屏障，类似asm volatile("" : : : "memory");
    static inline void clear_bit_unlock(unsigned nr, volatile unsigned long *addr)
    {
           barrier();
           clear_bit(nr, addr);
    }