When a real-time system is realized on a shared-memory multiprocessor, priority-ordered spin locks are often required to reduce priority inversions. However, simple priority-ordered spin locks can cause uncontrolled priority inversions when they are used for nested spin locks. This paper points out the problem of uncontrolled priority inversions in the context of spin locks and proposes priority inheritance spin locks, spin lock algorithms that are enhanced with the priority inheritance scheme, to solve the problem. Two algorithms of priority inheritance spin locks are presented and their effectiveness is demonstrated through performance measurements.