On Wed, Aug 24, 2022 at 10:16:10PM +0900, Kim Suwan wrote:

Hi Stefan,

On Wed, Aug 24, 2022 at 5:56 AM Stefan Hajnoczi [off-list ref] wrote:

quoted

On Tue, Aug 23, 2022 at 11:50:05PM +0900, Suwan Kim wrote:

quoted

@@ -409,6 +409,8 @@ static bool virtblk_add_req_batch(struct virtio_blk_vq *vq,
                      virtblk_unmap_data(req, vbr);
                      virtblk_cleanup_cmd(req);
                      rq_list_add(requeue_list, req);
+             } else {
+                     blk_mq_start_request(req);
              }

The device may see new requests as soon as virtblk_add_req() is called
above. Therefore the device may complete the request before
blk_mq_start_request() is called.

rq->io_start_time_ns = ktime_get_ns() will be after the request was
actually submitted.

I think blk_mq_start_request() needs to be called before
virtblk_add_req().

But if blk_mq_start_request() is called before virtblk_add_req()
and virtblk_add_req() fails, it can trigger WARN_ON_ONCE() at
virtio_queue_rq().

With regard to the race condition between virtblk_add_req() and
completion, I think that the race condition can not happen because
virtblk_add_req() holds vq lock with irq saving and completion side
(virtblk_done, virtblk_poll) need to acquire the vq lock also.
Moreover, virtblk_done() is irq context so I think it can not be
executed until virtblk_add_req() releases the lock.

I agree there is no race condition regarding the ordering of
blk_mq_start_request() and request completion. The spinlock prevents
that and I wasn't concerned about that part.

The issue is that the timestamp will be garbage. If we capture the
timestamp during/after the request is executing, then the collected
statistics will be wrong.

Can you look for another solution that doesn't break the timestamp?

FWIW I see that the rq->state state machine allows returning to the idle
state once the request has been started: __blk_mq_requeue_request().

Stefan