blktap: don't let in-flight requests defer pending ones

Running RING_FINAL_CHECK_FOR_REQUESTS from make_response is a bad
idea. It means that in-flight I/O is essentially blocking continued
batches. This essentially kills throughput on frontends which unplug
(or even just notify) early and rightfully assume additional requests
will be picked up on time, not synchronously.

Derived from a similar blkback patch by Daniel Stodden (see c/s
1118:c7c14595c18b).

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/drivers/xen/blktap/blktap.c
+++ b/drivers/xen/blktap/blktap.c
@@ -1285,7 +1285,7 @@ irqreturn_t tap_blkif_be_int(int irq, vo
  * DOWNWARD CALLS -- These interface with the block-device layer proper.
  */
 static int print_dbug = 1;
-static int do_block_io_op(blkif_t *blkif)
+static int _do_block_io_op(blkif_t *blkif)
 {
 	blkif_back_rings_t *blk_rings = &blkif->blk_rings;
 	blkif_request_t req;
@@ -1397,6 +1397,22 @@ static int do_block_io_op(blkif_t *blkif
 	return more_to_do;
 }
 
+static int do_block_io_op(blkif_t *blkif)
+{
+	blkif_back_rings_t *blk_rings = &blkif->blk_rings;
+	int more_to_do;
+
+	do {
+		more_to_do = _do_block_io_op(blkif);
+		if (more_to_do)
+			break;
+
+		RING_FINAL_CHECK_FOR_REQUESTS(&blk_rings->common, more_to_do);
+	} while (more_to_do);
+
+	return more_to_do;
+}
+
 static void dispatch_rw_block_io(blkif_t *blkif,
 				 blkif_request_t *req,
 				 pending_req_t *pending_req)
@@ -1647,7 +1663,6 @@ static void make_response(blkif_t *blkif
 	blkif_response_t  resp;
 	unsigned long     flags;
 	blkif_back_rings_t *blk_rings = &blkif->blk_rings;
-	int more_to_do = 0;
 	int notify;
 
 	resp.id        = id;
@@ -1678,20 +1693,7 @@ static void make_response(blkif_t *blkif
 	blk_rings->common.rsp_prod_pvt++;
 	RING_PUSH_RESPONSES_AND_CHECK_NOTIFY(&blk_rings->common, notify);
 
-	if (blk_rings->common.rsp_prod_pvt == blk_rings->common.req_cons) {
-		/*
-		 * Tail check for pending requests. Allows frontend to avoid
-		 * notifications if requests are already in flight (lower
-		 * overheads and promotes batching).
-		 */
-		RING_FINAL_CHECK_FOR_REQUESTS(&blk_rings->common, more_to_do);
-	} else if (RING_HAS_UNCONSUMED_REQUESTS(&blk_rings->common)) {
-		more_to_do = 1;
-	}
-
 	spin_unlock_irqrestore(&blkif->blk_ring_lock, flags);
-	if (more_to_do)
-		blkif_notify_work(blkif);
 	if (notify)
 		notify_remote_via_irq(blkif->irq);
 }