blk_end_request: add callback feature (take 4)

This patch adds a variant of the interface, blk_end_request_callback(),
which has driver callback feature.

Drivers may need to do special works between end_that_request_first()
and end_that_request_last().
For such drivers, blk_end_request_callback() allows it to pass
a callback function which is called between end_that_request_first()
and end_that_request_last().

This interface is only for fallback of other blk_end_request interfaces.
Drivers should avoid their tricky behaviors and use other interfaces
as much as possible.

Currently, only one driver, ide-cd, needs this interface.
So this interface should/will be removed, after the driver removes
such tricky behaviors.

o ide-cd (cdrom_newpc_intr())
  In PIO mode, cdrom_newpc_intr() needs to defer end_that_request_last()
  until the device clears DRQ_STAT and raises an interrupt after
  end_that_request_first().
  So end_that_request_first() and end_that_request_last() are called
  separately in cdrom_newpc_intr().

  This means blk_end_request_callback() has to return without
  completing request even if no leftover in the request.
  To satisfy the requirement, callback function has return value
  so that drivers can tell blk_end_request_callback() to return
  without completing request.

Signed-off-by: Kiyoshi Ueda <k-ueda@ct.jp.nec.com>
Signed-off-by: Jun'ichi Nomura <j-nomura@ce.jp.nec.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

block/ll_rw_blk.c

@@ -3825,10 +3825,14 @@ static void complete_request(struct request *rq, int error)
 }
 
 /**
- * blk_end_request - Helper function for drivers to complete the request.
+ * blk_end_io - Generic end_io function to complete a request.
- * @rq:       the request being processed
+ * @rq:           the request being processed
- * @error:    0 for success, < 0 for error
+ * @error:        0 for success, < 0 for error
- * @nr_bytes: number of bytes to complete
+ * @nr_bytes:     number of bytes to complete
+ * @drv_callback: function called between completion of bios in the request
+ *                and completion of the request.
+ *                If the callback returns non 0, this helper returns without
+ *                completion of the request.
  *
  * Description:
  *     Ends I/O on a number of bytes attached to @rq.
@@ -3836,9 +3840,10 @@ static void complete_request(struct request *rq, int error)
  *
  * Return:
  *     0 - we are done with this request
- *     1 - still buffers pending for this request
+ *     1 - this request is not freed yet, it still has pending buffers.
  **/
-int blk_end_request(struct request *rq, int error, int nr_bytes)
+static int blk_end_io(struct request *rq, int error, int nr_bytes,
+		      int (drv_callback)(struct request *))
 {
 	struct request_queue *q = rq->q;
 	unsigned long flags = 0UL;
@@ -3855,6 +3860,10 @@ int blk_end_request(struct request *rq, int error, int nr_bytes)
 			return 1;
 	}
 
+	/* Special feature for tricky drivers */
+	if (drv_callback && drv_callback(rq))
+		return 1;
+
 	add_disk_randomness(rq->rq_disk);
 
 	spin_lock_irqsave(q->queue_lock, flags);
@@ -3863,6 +3872,25 @@ int blk_end_request(struct request *rq, int error, int nr_bytes)
 
 	return 0;
 }
+
+/**
+ * blk_end_request - Helper function for drivers to complete the request.
+ * @rq:       the request being processed
+ * @error:    0 for success, < 0 for error
+ * @nr_bytes: number of bytes to complete
+ *
+ * Description:
+ *     Ends I/O on a number of bytes attached to @rq.
+ *     If @rq has leftover, sets it up for the next range of segments.
+ *
+ * Return:
+ *     0 - we are done with this request
+ *     1 - still buffers pending for this request
+ **/
+int blk_end_request(struct request *rq, int error, int nr_bytes)
+{
+	return blk_end_io(rq, error, nr_bytes, NULL);
+}
 EXPORT_SYMBOL_GPL(blk_end_request);
 
 /**
@@ -3901,6 +3929,38 @@ int __blk_end_request(struct request *rq, int error, int nr_bytes)
 }
 EXPORT_SYMBOL_GPL(__blk_end_request);
 
+/**
+ * blk_end_request_callback - Special helper function for tricky drivers
+ * @rq:           the request being processed
+ * @error:        0 for success, < 0 for error
+ * @nr_bytes:     number of bytes to complete
+ * @drv_callback: function called between completion of bios in the request
+ *                and completion of the request.
+ *                If the callback returns non 0, this helper returns without
+ *                completion of the request.
+ *
+ * Description:
+ *     Ends I/O on a number of bytes attached to @rq.
+ *     If @rq has leftover, sets it up for the next range of segments.
+ *
+ *     This special helper function is used only for existing tricky drivers.
+ *     (e.g. cdrom_newpc_intr() of ide-cd)
+ *     This interface will be removed when such drivers are rewritten.
+ *     Don't use this interface in other places anymore.
+ *
+ * Return:
+ *     0 - we are done with this request
+ *     1 - this request is not freed yet.
+ *         this request still has pending buffers or
+ *         the driver doesn't want to finish this request yet.
+ **/
+int blk_end_request_callback(struct request *rq, int error, int nr_bytes,
+			     int (drv_callback)(struct request *))
+{
+	return blk_end_io(rq, error, nr_bytes, drv_callback);
+}
+EXPORT_SYMBOL_GPL(blk_end_request_callback);
+
 static void blk_rq_bio_prep(struct request_queue *q, struct request *rq,
 			    struct bio *bio)
 {

include/linux/blkdev.h

@@ -734,6 +734,8 @@ extern void end_that_request_last(struct request *, int);
 extern void end_request(struct request *, int);
 extern void end_queued_request(struct request *, int);
 extern void end_dequeued_request(struct request *, int);
+extern int blk_end_request_callback(struct request *rq, int error, int nr_bytes,
+				    int (drv_callback)(struct request *));
 extern void blk_complete_request(struct request *);
 
 /*