down_write(&md->lock);
down_write(&md->io_lock);
static int queue_io(struct mapped_device *md, struct bio *bio){	down_write(&md->lock);	if (!test_bit(DMF_BLOCK_IO, &md->flags)) {		up_write(&md->lock);		return 1;	}	bio_list_add(&md->deferred, bio);	up_write(&md->lock);	return 0;		/* deferred successfully */}