Hi Bart

Thanks for your comment on this.

On 3/19/19 1:16 AM, Bart Van Assche wrote:

On Fri, 2019-03-15 at 16:57 +0800, Jianchao Wang wrote:

quoted

blk_mq_tagset_busy_iter is not safe that it could get stale request
in tags->rqs[]. Use blk_mq_queue_tag_busy_iter here.

Signed-off-by: Jianchao Wang <redacted>
---
 drivers/block/nbd.c | 2 +
 1 file changed, 1 insertion(), 1 deletion(-)

diff --git a/drivers/block/nbd.c b/drivers/block/nbd.c
index 7c9a949..9e7e828 100644
--- a/drivers/block/nbd.c
+++ b/drivers/block/nbd.c

@@ -747,7 +747,7 @@ static bool nbd_clear_req(struct request *req, void *data, bool reserved)
 static void nbd_clear_que(struct nbd_device *nbd)
 {
        blk_mq_quiesce_queue(nbd->disk->queue);
-       blk_mq_tagset_busy_iter(&nbd->tag_set, nbd_clear_req, NULL);
+       blk_mq_queue_tag_busy_iter(nbd->disk->queue, nbd_clear_req, NULL, true);
        blk_mq_unquiesce_queue(nbd->disk->queue);
        dev_dbg(disk_to_dev(nbd->disk), "queue cleared\n");
 }

Hi Jianchao,

The nbd driver calls nbd_clear_que() after having called sock_shutdown(). So
what makes you think that it's not safe to call blk_mq_tagset_busy_iter()
from nbd_clear_que()?

The request_queue is not frozen, so there still could be someone enters and get
request. If no io scheduler attached, the driver tag could be allocated and 
tags->rqs[] would be set. During this, blk_mq_tagset_busy_iter could get some stale
requests in tags->rqs[] which maybe freed due to switching io scheduler to none.

Thanks
Jianchao