block: only call ->request_fn when the queue is not stopped

Callers should use either blk_run_queue/__blk_run_queue, or
blk_start_queueing() to invoke request handling instead of calling
->request_fn() directly as that does not take the queue stopped
flag into account.

Also add appropriate comments on the above functions to detail
their usage.
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

block/blk-core.c

@@ -325,6 +325,9 @@ EXPORT_SYMBOL(blk_unplug);
 
 static void blk_invoke_request_fn(struct request_queue *q)
 {
+	if (unlikely(blk_queue_stopped(q)))
+		return;
+
 	/*
 	 * one level of recursion is ok and is much faster than kicking
 	 * the unplug handling
@@ -399,8 +402,13 @@ void blk_sync_queue(struct request_queue *q)
 EXPORT_SYMBOL(blk_sync_queue);
 
 /**
- * blk_run_queue - run a single device queue
+ * __blk_run_queue - run a single device queue
  * @q:	The queue to run
+ *
+ * Description:
+ *    See @blk_run_queue. This variant must be called with the queue lock
+ *    held and interrupts disabled.
+ *
  */
 void __blk_run_queue(struct request_queue *q)
 {
@@ -418,6 +426,12 @@ EXPORT_SYMBOL(__blk_run_queue);
 /**
  * blk_run_queue - run a single device queue
  * @q: The queue to run
+ *
+ * Description:
+ *    Invoke request handling on this queue, if it has pending work to do.
+ *    May be used to restart queueing when a request has completed. Also
+ *    See @blk_start_queueing.
+ *
  */
 void blk_run_queue(struct request_queue *q)
 {
@@ -884,7 +898,8 @@ EXPORT_SYMBOL(blk_get_request);
  *
  * This is basically a helper to remove the need to know whether a queue
  * is plugged or not if someone just wants to initiate dispatch of requests
- * for this queue.
+ * for this queue. Should be used to start queueing on a device outside
+ * of ->request_fn() context. Also see @blk_run_queue.
  *
  * The queue lock must be held with interrupts disabled.
  */

block/elevator.c

@@ -612,7 +612,7 @@ void elv_insert(struct request_queue *q, struct request *rq, int where)
 		 *   processing.
 		 */
 		blk_remove_plug(q);
-		q->request_fn(q);
+		blk_start_queueing(q);
 		break;
 
 	case ELEVATOR_INSERT_SORT:
@@ -950,7 +950,7 @@ void elv_completed_request(struct request_queue *q, struct request *rq)
 		    blk_ordered_cur_seq(q) == QUEUE_ORDSEQ_DRAIN &&
 		    blk_ordered_req_seq(first_rq) > QUEUE_ORDSEQ_DRAIN) {
 			blk_ordered_complete_seq(q, QUEUE_ORDSEQ_DRAIN, 0);
-			q->request_fn(q);
+			blk_start_queueing(q);
 		}
 	}
 }
@@ -1109,8 +1109,7 @@ static int elevator_switch(struct request_queue *q, struct elevator_type *new_e)
 	elv_drain_elevator(q);
 
 	while (q->rq.elvpriv) {
-		blk_remove_plug(q);
-		q->request_fn(q);
+		blk_start_queueing(q);
 		spin_unlock_irq(q->queue_lock);
 		msleep(10);
 		spin_lock_irq(q->queue_lock);