Re: [PATCH 4/5] dm: implement REQ_FLUSH/FUA support for request-based dm

[Tejun Heo <tj@kernel.org>]

On 08/30/2010 03:59 PM, Tejun Heo wrote:
> Ah... that's probably from "if (!elv_queue_empty(q))" check below,
> flushes are on a separate queue but I forgot to update
> elv_queue_empty() to check the flush queue.  elv_queue_empty() can
> return %true spuriously in which case the queue won't be plugged and
> restarted later leading to queue hang.  I'll fix elv_queue_empty().

I think I was too quick to blame elv_queue_empty().  Can you please
test whether the following patch fixes the hang?

Thanks.

---
 block/blk-flush.c |   18 +++++++++++++-----
 1 file changed, 13 insertions(+), 5 deletions(-)

Index: block/block/blk-flush.c
===================================================================
--- block.orig/block/blk-flush.c
+++ block/block/blk-flush.c
@@ -28,7 +28,8 @@ unsigned blk_flush_cur_seq(struct reques
 }

 static struct request *blk_flush_complete_seq(struct request_queue *q,
-					      unsigned seq, int error)
+					      unsigned seq, int error,
+					      bool from_end_io)
 {
 	struct request *next_rq = NULL;

@@ -51,6 +52,13 @@ static struct request *blk_flush_complet
 		if (!list_empty(&q->pending_flushes)) {
 			next_rq = list_entry_rq(q->pending_flushes.next);
 			list_move(&next_rq->queuelist, &q->queue_head);
+			/*
+			 * Moving a request silently to queue_head may
+			 * stall the queue, kick the queue if we
+			 * aren't in the issue path already.
+			 */
+			if (from_end_io)
+				__blk_run_queue(q);
 		}
 	}
 	return next_rq;
@@ -59,19 +67,19 @@ static struct request *blk_flush_complet
 static void pre_flush_end_io(struct request *rq, int error)
 {
 	elv_completed_request(rq->q, rq);
-	blk_flush_complete_seq(rq->q, QUEUE_FSEQ_PREFLUSH, error);
+	blk_flush_complete_seq(rq->q, QUEUE_FSEQ_PREFLUSH, error, true);
 }

 static void flush_data_end_io(struct request *rq, int error)
 {
 	elv_completed_request(rq->q, rq);
-	blk_flush_complete_seq(rq->q, QUEUE_FSEQ_DATA, error);
+	blk_flush_complete_seq(rq->q, QUEUE_FSEQ_DATA, error, true);
 }

 static void post_flush_end_io(struct request *rq, int error)
 {
 	elv_completed_request(rq->q, rq);
-	blk_flush_complete_seq(rq->q, QUEUE_FSEQ_POSTFLUSH, error);
+	blk_flush_complete_seq(rq->q, QUEUE_FSEQ_POSTFLUSH, error, true);
 }

 static void init_flush_request(struct request *rq, struct gendisk *disk)
@@ -165,7 +173,7 @@ struct request *blk_do_flush(struct requ
 		skip |= QUEUE_FSEQ_DATA;
 	if (!do_postflush)
 		skip |= QUEUE_FSEQ_POSTFLUSH;
-	return blk_flush_complete_seq(q, skip, 0);
+	return blk_flush_complete_seq(q, skip, 0, false);
 }

 static void bio_end_flush(struct bio *bio, int err)