Man page 9 : blk_release_queue

blk_release_queue - release a struct request_queue when it is no longer needed

void blk_release_queue(struct kobject *  kobj );

kobj
    the kobj belonging to the request queue to be released

blk_release_queue is the pair to blk_init_queue or blk_queue_make_request. It should be called when a request queue is being released; typically when a block device is being de-registered. Currently, its primary task it to free all the struct request structures that were allocated to the queue and the queue itself.

Hopefully the low level driver will have finished any outstanding requests first...