How can a spinlock progress when it's busy-waiting?
There might be other CPUs in the system, if one is busy waiting, another can be doing something. Furthermore, if the OS uses preemptive scheduling, the thread doing the busy wait might be preempted and another thread will do something and release the lock for example. The signal might also come from an interrupt handler, for example if the thread is waiting for the completion of some I/O operation, and an interrupt will stop the execution of the thread temporarily, signal the lock, and resume the thread.