On Spin Locks in AUTOSAR: Blocking Analysis of FIFO, Unordered, and Priority-Ordered Spin Locks

Abstract

Motivated by the widespread use of spin locks in embedded multiprocessor real-time systems, the worst-case blocking in spin locks is analyzed using mixed-integer linear programming. Four queue orders and two preemption models are studied: (i) FIFO-ordered spin locks, (ii) unordered spin locks, (iii) priority-ordered spin locks with unordered tie-breaking, and (iv) priority-ordered spin locks with FIFO-ordered tie-breaking, each analyzed assuming both preempt able and non-preempt able spinning. Of the eight lock types, seven have not been analyzed in prior work. Concerning the sole exception (non-preempt able FIFO spin locks), the new analysis is asymptotically less pessimistic and typically much more accurate since no critical section is accounted for more than once. The eight lock types are empirically compared in schedulability experiments. While the presented analysis is generic in nature and applicable to real-time systems in general, it is specifically motivated by the recent inclusion of spin locks into the AUTOSAR standard, and four concrete suggestions for an improved AUTOSAR spin lock API are derived from the results.