/usr/lib/ruby/ is in ruby-atomic 1.1.16-1ubuntu1.
This file is owned by root:root, with mode 0o755.
..
/usr/lib/ruby/vendor_ruby/atomic/fallback.rb
/usr/lib/ruby/vendor_ruby/
/usr/lib/ruby/vendor_ruby/1.9.1/
/usr/lib/ruby/vendor_ruby/1.9.1/x86_64-linux/
/usr/lib/ruby/vendor_ruby/1.9.1/x86_64-linux/atomic_reference.so
/usr/lib/ruby/vendor_ruby/atomic/
/usr/lib/ruby/vendor_ruby/atomic/direct_update.rb
/usr/lib/ruby/vendor_ruby/atomic/delegated_update.rb
/usr/lib/ruby/vendor_ruby/atomic/jruby.rb
/usr/lib/ruby/vendor_ruby/atomic/numeric_cas_wrapper.rb
/usr/lib/ruby/vendor_ruby/atomic/ruby.rb
/usr/lib/ruby/vendor_ruby/atomic/rbx.rb
/usr/lib/ruby/vendor_ruby/atomic/concurrent_update_error.rb
/usr/lib/ruby/vendor_ruby/atomic.rb