The InnoDB custom implementation of a read-write latch (rw_lock_t) encapsulates two os_event_t. Both os_event_t encapsulate a mutex and a condition variable. One mutex would suffice. There also are some data fields to control the waiting, although the rw_lock_t::lock_word alone is sufficient for that.
As part of
MDEV-21452, we would replace all os_event_t with condition variables. Simplifying the read-write latch implementation is a prerequisite for that.