[PATCH] xen-blkfront: Fix IO race during unplug

[Ross Lagerwall]

During unplug, blkfront stops the hw queues and marks the disk as dead,
then later during removal calls del_gendisk(). However, IO issued after
the hw queues are stopped but before the call to del_gendisk() will be
queued but never handled. This causes del_gendisk() to hang forever
waiting for the queue refcount to drop to zero.

This can be reproduced by issuing IO during an artificial delay after
stopping the hw queues.

Fix this by simply not stopping the hw queues directly. Marking the disk
as dead also freezes the queue which prevents new requests being added
and it synchronously runs the hw queues to clear anything pending.

Fixes: 8e141f9eb803 ("block: drain file system I/O on del_gendisk")
Cc: stable@xxxxxxxxxxxxxxx
Assisted-by: LLM
Signed-off-by: Ross Lagerwall <ross.lagerwall@xxxxxxxxxx>
---

I'm not sure about the Fixes tag. It's the most likely looking candidate
to me but I didn't confirm whether it actually introduced the
regression.

 drivers/block/xen-blkfront.c | 4 +---
 1 file changed, 1 insertion(+), 3 deletions(-)

diff --git a/drivers/block/xen-blkfront.c b/drivers/block/xen-blkfront.c
index 8dad7bf5f664..69a2315a1b20 100644
--- a/drivers/block/xen-blkfront.c
+++ b/drivers/block/xen-blkfront.c
@@ -2138,10 +2138,8 @@ static void blkfront_closing(struct blkfront_info *info)
 		return;
 
 	/* No more blkif_request(). */
-	if (info->rq && info->gd) {
-		blk_mq_stop_hw_queues(info->rq);
+	if (info->gd)
 		blk_mark_disk_dead(info->gd);
-	}
 
 	for_each_rinfo(info, rinfo, i) {
 		/* No more gnttab callback work. */
-- 
2.55.0