Re: [PATCH 3/3] blk-mq: fix wait condition for tagset wait completed check

Nilay Shroff
2025-01-29T09:55:17Z

On 1/28/25 10:04 PM, Daniel Wagner wrote:
> blk_mq_tagset_count_completed_reqs returns the number of completed
> requests. The only user of this function is
> blk_mq_tagset_wait_completed_request which wants to know how many
> request are not yet completed. Thus return the number of in flight
> requests and terminate the wait loop when there is no inflight request.
>
> Fixes: f9934a80f91d ("blk-mq: introduce blk_mq_tagset_wait_completed_request()")
> Cc: Ming Lei <ming.lei@redhat.com>
> Signed-off-by: Daniel Wagner <wagi@kernel.org>
> ---
> block/blk-mq-tag.c | 6 +++---
> 1 file changed, 3 insertions(+), 3 deletions(-)
>
> diff --git a/block/blk-mq-tag.c b/block/blk-mq-tag.c
> index b9f417d980b46d54b74dec8adcb5b04e6a78635c..3ce46afb65e3c3de9f11ca440bf0f335f21d0998 100644
> --- a/block/blk-mq-tag.c
> +++ b/block/blk-mq-tag.c
> @@ -450,11 +450,11 @@ void blk_mq_tagset_busy_iter(struct blk_mq_tag_set *tagset,
> }
> EXPORT_SYMBOL(blk_mq_tagset_busy_iter);
>
> -static bool blk_mq_tagset_count_completed_rqs(struct request *rq, void *data)
> +static bool blk_mq_tagset_count_inflight_rqs(struct request *rq, void *data)
> {
> unsigned *count = data;
>
> - if (blk_mq_request_completed(rq))
> + if (blk_mq_rq_state(rq) == MQ_RQ_IN_FLIGHT)
> (*count)++;
> return true;
> }
> @@ -472,7 +472,7 @@ void blk_mq_tagset_wait_completed_request(struct blk_mq_tag_set *tagset)
> unsigned count = 0;
>
> blk_mq_tagset_busy_iter(tagset,
> - blk_mq_tagset_count_completed_rqs, &count);
> + blk_mq_tagset_count_inflight_rqs, &count);
> if (!count)
> break;
> msleep(5);
>
I see that blk_mq_tagset_wait_completed_request() is called from nvme_cancel_tagset()
and nvme_cancel_admin_tagset(). And it seems to me that the intention here's to wait
until each completed requests are freed (or change its state to MQ_RQ_IDLE).

Looking at code, the nvme_cancel_xxx() first invokes blk_mq_tagset_busy_iter() which
iterates through tagset and cancels each in-flight request and marks the request state
to MQ_RQ_COMPLETE. Next in blk_mq_tagset_wait_completed_request(), we wait for each
completed request state changed to anything but MQ_RQ_COMPLETE. The next state of the
request would be naturally MQ_RQ_IDLE once that request is freed. So in blk_mq_tagset_
wait_completed_request(), essentially we wait for request state change from MQ_RQ_COMPLETE
to MQ_RQ_IDLE.

So now if the proposal is that blk_mq_tagset_wait_completed_request() has to wait only
if there's any in-flight request then it seems this function would never need to wait
and looks redundant because req->state would never be MQ_RQ_IN_FLIGHT as those would
have been already changed to MQ_RQ_COMPLETE when nvme_cancel_xxx() invokes blk_mq_tagset_
busy_iter(ctrl->tagset, nvme_cancel_request, ctrl).

Having said that, I am not sure what was the real intention here, in nvme_cancel_xxx(),
do we really need to wait only until in-flight requests are completed or synchronize with
request's completion callback (i.e. wait until all completed requests are freed)?

Thanks,
--Nilay