directio: add block polling support

This adds support for sync O_DIRECT read/write poll support.
Signed-off-by: Jens Axboe <axboe@fb.com>
[hch: split from a larger patch, minor updates]
Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Keith Busch <keith.busch@intel.com>

fs/direct-io.c

@@ -109,6 +109,8 @@ struct dio_submit {
 struct dio {
 	int flags;			/* doesn't change */
 	int rw;
+	blk_qc_t bio_cookie;
+	struct block_device *bio_bdev;
 	struct inode *inode;
 	loff_t i_size;			/* i_size when submitted */
 	dio_iodone_t *end_io;		/* IO completion function */
@@ -397,11 +399,14 @@ static inline void dio_bio_submit(struct dio *dio, struct dio_submit *sdio)
 	if (dio->is_async && dio->rw == READ && dio->should_dirty)
 		bio_set_pages_dirty(bio);
 
-	if (sdio->submit_io)
+	if (sdio->submit_io) {
 		sdio->submit_io(dio->rw, bio, dio->inode,
 			       sdio->logical_offset_in_bio);
-	else
-		submit_bio(dio->rw, bio);
+		dio->bio_cookie = BLK_QC_T_NONE;
+	} else {
+		dio->bio_cookie = submit_bio(dio->rw, bio);
+		dio->bio_bdev = bio->bi_bdev;
+	}
 
 	sdio->bio = NULL;
 	sdio->boundary = 0;
@@ -440,7 +445,8 @@ static struct bio *dio_await_one(struct dio *dio)
 		__set_current_state(TASK_UNINTERRUPTIBLE);
 		dio->waiter = current;
 		spin_unlock_irqrestore(&dio->bio_lock, flags);
-		io_schedule();
+		if (!blk_poll(bdev_get_queue(dio->bio_bdev), dio->bio_cookie))
+			io_schedule();
 		/* wake up sets us TASK_RUNNING */
 		spin_lock_irqsave(&dio->bio_lock, flags);
 		dio->waiter = NULL;