> the other process that's spinning on the lock can acquire lock.

> but in some implementations of spinlock, like qspinlock in x86 (or mcs lock),
> I think there's no need to disable preemption. because processes
> waiting for lock cannot acquire the lock before the lock holder hand
> over to other process.

Are you talking about disabling local irq(ex. spin_lock_irqsave)?

If so, think about the situation that a process holding the lock is preempted 

by interrupt. And that interrupt handler tries to grab the spinlock.  

It will lead to deadlock.