为什么linux在内核代码持有自旋锁后禁用内核抢占？

If a nonpreemptive uniprocessor system ever went into a
spin on a lock,it would spin forever; no other thread would ever be able to obtain
the CPU to release the lock. For this reason,spinlock operations on uniprocessor systems
without preemption enabled are optimized to do nothing,with the exception of
the ones that change the IRQ masking status.

书还说明了这一点

The kernel preemption case is handled by the spinlock code itself. Any time kernel
code holds a spinlock,preemption is disabled on the relevant processor. Even uniprocessor
systems must disable preemption in this way to avoid race conditions.

问题：在单处理器系统上,如果内核代码(代表用户进程执行)持有自旋锁,则禁用内核抢占,那么另一个进程怎么能有机会运行并因此尝试获取自旋锁？为什么Linux内核会在内核代码持有自旋锁时禁用内核抢占？