Flow along a curved wall is susceptible to centrifugal instability. The criterion of Synge [Proc. R. Soc. London Ser. A 167, 250 (1938)] for the stability of flow between rotating cylinders is generalized to the case of an arbitrary flow along a convex wall. Sufficient conditions for the linear stability of the flow are given, based on the behavior of a function similar to the Rayleigh discriminant.