In a mixing bowl, combine flour and butter for the crust. Add ice water and mix until dough forms. Divide dough in half and roll out each half to fit a 9-inch pie dish.
Place one rolled-out dough in the pie dish. In another bowl, mix rhubarb, sugar, flour, and cinnamon. Pour mixture into the pie crust and dot with butter.
Cover with the second rolled-out dough, seal edges, and cut small slits on top to vent. Bake for 50 minutes or until crust is golden brown and filling is bubbly.