[PATCH 1/2] block: mq-deadline: Fix dd_finish_request() for zoned devices

[Damien Le Moal <damien.lemoal@opensource.wdc.com>]

dd_finish_request() tests if the per prio fifo_list is not empty to
determine if request dispatching must be restarted for handling blocked
write requests to zoned devices with a call to
blk_mq_sched_mark_restart_hctx(). While simple, this implementation has
2 problems:

1) Only the priority level of the completed request is considered.
   However, writes to a zone may be blocked due to other writes to the
   same zone using a different priority level. While this is unlikely to
   happen in practice, as writing a zone with different IO priorirites
   does not make sense, nothing in the code prevents this from
   happening.
2) The use of list_empty() is dangerous as dd_finish_request() does not
   take dd->lock and may run concurrently with the insert and dispatch
   code.

Fix these 2 problems by testing the write fifo list of all priority
levels using the new helper dd_has_write_work(), and by testing each
fifo list using list_empty_careful().

Fixes: c807ab520fc3 ("block/mq-deadline: Add I/O priority support")
Cc: <stable@vger.kernel.org>
Signed-off-by: Damien Le Moal <damien.lemoal@opensource.wdc.com>
---
 block/mq-deadline.c | 17 +++++++++++++++--
 1 file changed, 15 insertions(+), 2 deletions(-)

diff --git a/block/mq-deadline.c b/block/mq-deadline.c
index 5639921dfa92..36374481cb87 100644
--- a/block/mq-deadline.c
+++ b/block/mq-deadline.c
@@ -789,6 +789,18 @@ static void dd_prepare_request(struct request *rq)
 	rq->elv.priv[0] = NULL;
 }
 
+static bool dd_has_write_work(struct blk_mq_hw_ctx *hctx)
+{
+	struct deadline_data *dd = hctx->queue->elevator->elevator_data;
+	enum dd_prio p;
+
+	for (p = 0; p <= DD_PRIO_MAX; p++)
+		if (!list_empty_careful(&dd->per_prio[p].fifo_list[DD_WRITE]))
+			return true;
+
+	return false;
+}
+
 /*
  * Callback from inside blk_mq_free_request().
  *
@@ -828,9 +840,10 @@ static void dd_finish_request(struct request *rq)
 
 		spin_lock_irqsave(&dd->zone_lock, flags);
 		blk_req_zone_write_unlock(rq);
-		if (!list_empty(&per_prio->fifo_list[DD_WRITE]))
-			blk_mq_sched_mark_restart_hctx(rq->mq_hctx);
 		spin_unlock_irqrestore(&dd->zone_lock, flags);
+
+		if (dd_has_write_work(rq->mq_hctx))
+			blk_mq_sched_mark_restart_hctx(rq->mq_hctx);
 	}
 }
 
-- 
2.38.1