Detecting data races in Cilk programs that use locks

Abstract

When two parallel threads holding no locks in common access the same memory location and at least one of the threads modifies the location, a “data race” occurs, which is usually a bug. This paper describes the algorithms and strategies used by a debugging tool, called the Nondeterminator-2, which checks for data races in programs coded in the Cilk multithreaded language. Like its predecessor, the Nondeterminator, which checks for simple “determinacy” races, the Nondeterminator-2 is a debugging tool, not a verifier, since it checks for data races only in the computation generated by a serial execution of the program on a given input. We give an algorithm, ALL-SETS, that determines whether the computation generated by a serial execution of a Cilk program on a given input contains a race. For a program that runs serially in time T , accesses V shared memory locations, uses a total of n locks, and holds at most k n locks simultaneously, ALL-SETS runs in O(nkT α(V;V )) time and O(nkV ) space, where α is Tarjan’s functional inverse of Ackermann’s function. Since ALL-SETS may be too inefficient in the worst case, we propose a much more efficient algorithm which can be used to detect races in programs that obey the “umbrella” locking discipline, a programming methodology that is more flexible than similar disciplines proposed in the literature. We present an algorithm, BRELLY, which detects violations of the umbrella discipline in O(kT α(V;V )) time using O(kV ) space. We also prove that any “abelian” Cilk program, one whose critical sections commute, produces a determinate final state if it is deadlock free and if it generates any computation which is datarace free. Thus, the Nondeterminator-2’s two algorithms can verify the determinacy of a deadlock-free abelian program running on a given input. Keywords