In InnoDB, there is a configuration parameter innodb_instrument_semaphores. The parameter seems to only affect the rw-lock X-latch acquisition. Extra fields are added to rw_lock_t to remember one X-latch holder or waiter. These fields are not being consulted or reported anywhere. This is basically only adding code bloat.

If the intention is to debug hangs or deadlocks, we have better tools for that in the debug server, and for the non-debug server, core dumps can reveal a lot. For example, the mini-transaction memo records the currently held buffer block or index rw-locks, to be released at mtr_t::commit().

The configuration parameter innodb_instrument_semaphores should be deprecated in 10.2.5 and removed in 10.3.0.