  Energy extraction from a rotating or charged black hole is one of fascinating
issues in general relativity. The collisional Penrose process is one of such
extraction mechanisms and has been reconsidered intensively since Banados, Silk
and West pointed out the physical importance of very high energy collisions
around a maximally rotating black hole. In order to get results analytically,
the test particle approximation has been adopted so far. Successive works based
on this approximation scheme have not yet revealed the upper bound on the
efficiency of the energy extraction because of lack of the back reaction. In
the Reissner-Nordstrom spacetime, by fully taking into account the self-gravity
of the shells, we find that there is an upper bound on the extracted energy,
which is consistent with the area law of a black hole. We also show one
particular scenario in which the almost maximum energy extraction is achieved
even without the Banados-Silk-West collision.
