For consistency with mongod --upgrade , mongos --upgrade should exit after upgrading. (Currently it upgrades and then forks if --fork is given and then starts accepting connections)
cc eliot firstname.lastname@example.org email@example.com
Option for 'mongos --upgrade' to exit after metadata upgrade is complete
During Upgrade of Mongos from 2.2 to 2.4, process does not come out.
update cluster upgrade instruction