Re: [PATCH] blk-mq: streamline handling of q->mq_ops->queue_rq result

[Christoph Hellwig <hch@infradead.org>]

On Wed, Jul 01, 2020 at 06:16:17PM +0800, Ming Lei wrote:
> Current handling of q->mq_ops->queue_rq result is a bit ugly:
> 
> - two branches which needs to 'continue' have to check if the
> dispatch local list is empty, otherwise one bad request may
> be retrieved via 'rq = list_first_entry(list, struct request, queuelist);'
> 
> - the branch of 'if (unlikely(ret != BLK_STS_OK))' isn't easy
> to follow, since it is actually one error branch.
> 
> Streamline this handling, so the code becomes more readable, meantime
> potential kernel oops can be avoided in case that the last request in
> local dispatch list is failed.

I don't really find that much easier to read.  If we want to clean
this up for rea we should use a proper switch statement.  Something like
this:

diff --git a/block/blk-mq.c b/block/blk-mq.c
index a9aa6d1e44cf32..f3721f274b800e 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -1275,30 +1275,28 @@ bool blk_mq_dispatch_rq_list(struct request_queue *q, struct list_head *list,
 		}
 
 		ret = q->mq_ops->queue_rq(hctx, &bd);
-		if (ret == BLK_STS_RESOURCE || ret == BLK_STS_DEV_RESOURCE) {
-			blk_mq_handle_dev_resource(rq, list);
+		switch (ret) {
+		case BLK_STS_OK:
+			queued++;
 			break;
-		} else if (ret == BLK_STS_ZONE_RESOURCE) {
+		case BLK_STS_RESOURCE:
+		case BLK_STS_DEV_RESOURCE:
+			blk_mq_handle_dev_resource(rq, list);
+			goto out;
+		case BLK_STS_ZONE_RESOURCE:
 			/*
 			 * Move the request to zone_list and keep going through
 			 * the dispatch list to find more requests the drive can
 			 * accept.
 			 */
 			blk_mq_handle_zone_resource(rq, &zone_list);
-			if (list_empty(list))
-				break;
-			continue;
-		}
-
-		if (unlikely(ret != BLK_STS_OK)) {
+			break;
+		default:
 			errors++;
 			blk_mq_end_request(rq, BLK_STS_IOERR);
-			continue;
 		}
-
-		queued++;
 	} while (!list_empty(list));
-
+out:
 	if (!list_empty(&zone_list))
 		list_splice_tail_init(&zone_list, list);