mdb_readers(3) on Linux
The text b1ce0c8949a5, as Debian 12 (Bookworm), Debian 11 (Bullseye) shipped it.
Readers don't acquire any locks for their data access. Instead, they simply record their transaction ID in the reader table. The reader mutex is needed just to find an empty slot in the reader table. The slot's address is saved in thread-specific data so that subsequent read transactions started by the same thread need no further locking to proceed.