m.Lock(id, source)
				acc0 -= 100
				acc1 += 100
				m.Unlock(context.Background())
			} else {
				for i := 0; i < len(data); i += 4 {
					data[i]++
				}
				// Elaborate way to say runtime.Gosched
				// that does not put the goroutine onto global runq.
				go func() {
					c <- true
				}()
				<-c
			}
		}
	})
}

func BenchmarkMutexSpin(b *testing.B) {
	b.ResetTimer()