Just checked the upstream code, looks like PS added RW-lock on buf_pool->page_hash_latch, both in buf_LRU_remove_all_pages and buf_page_get_gen (upstream doesn't have for those). This may have been introduced when btr_search_drop_page_hash_when_freed was added to newly added function buf_LRU_remove_all_pages which wasn't there earlier. (may have been part of 5.5.23 upstream DROP TABLE code)
Just checked the upstream code, looks like PS added RW-lock on buf_pool- >page_hash_ latch, both in buf_LRU_ remove_ all_pages and buf_page_get_gen (upstream doesn't have for those). This may have been introduced when btr_search_ drop_page_ hash_when_ freed was added to newly added function buf_LRU_ remove_ all_pages which wasn't there earlier. (may have been part of 5.5.23 upstream DROP TABLE code)