While there are many ways to roll an airplane that would cause complete havoc inside the cabin, a barrel roll can be performed as a "1g" maneuver. With careful control of the aircraft, the forces can be balanced so that you only ever feel close to 1g of apparent gravity, and it always is pulling you down into your seat.

This was famously completed by Tex Johnston, without authorization, in a prototype Boeing 707. As acrobatic maneuvers go it's very safe, because the stresses on the airframe are minimal. We could update the programming on any modern airliner to allow the pilot to trigger a barrel roll, and it could smoothly move the plane through the whole process.

A major reason we wouldn't do that today is it's not worth it for the manufacturer: it would be a lot of work to ensure the code was completely correct. I wonder if as programming, validation, and evaluation continue to get more automated the cost of adding this and getting it through regulatory approval would get low enough that we might see some airlines use this in marketing?