As part of the SPATIAL INDEX implementation in InnoDB,
dict_index_t was expanded by a rtr_ssn_t field. There are only
3 operations for this field, all protected by rtr_ssn_t::mutex:

* btr_cur_search_to_nth_level() stores the least significant 32 bits
of the 64-bit value that is stored in the index root page.
(This would better be done when the table is opened for the
very first time.)
* rtr_get_new_ssn_id() increments the value by 1.
* rtr_get_current_ssn_id() reads the current value.

All these operations can be implemented equally safely by using
atomic memory access operations.