int writeback_acquire (struct backing_dev_info * bdi);
It is a waste of resources to have more than one pdflush thread blocked on a single request queue. Exclusion at the request_queue level is obtained via a flag in the request_queue's backing_dev_info.state.
Non-request_queue-backed address_spaces will share default_backing_dev_info, unless they implement their own. Which is somewhat inefficient, as this may prevent concurrent writeback against multiple devices.