[PATCH] block: re-introduce blk_mq_complete_request_sync

[Chao Leng]

rdma also need do this patch.
We do test this patch with nvme over roce a few days, now work well.

On 2020/10/14 9:08, Ming Lei wrote:
> On Tue, Oct 13, 2020 at 03:36:08PM -0700, Sagi Grimberg wrote:
>>
>>>>> This may just reduce the probability. The concurrency of timeout
>>>>> and teardown will cause the same request
>>>>> be treated repeatly, this is not we expected.
>>>>
>>>> That is right, not like SCSI, NVME doesn't apply atomic request
>>>> completion, so
>>>> request may be completed/freed from both timeout & nvme_cancel_request().
>>>>
>>>> .teardown_lock still may cover the race with Sagi's patch because
>>>> teardown
>>>> actually cancels requests in sync style.
>>> In extreme scenarios, the request may be already retry success(rq state
>>> change to inflight).
>>> Timeout processing may wrongly stop the queue and abort the request.
>>> teardown_lock serialize the process of timeout and teardown, but do not
>>> avoid the race.
>>> It might not be safe.
>>
>> Not sure I understand the scenario you are describing.
>>
>> what do you mean by "In extreme scenarios, the request may be already retry
>> success(rq state change to inflight)"?
>>
>> What will retry the request? only when the host will reconnect
>> the request will be retried.
>>
>> We can call nvme_sync_queues in the last part of the teardown, but
>> I still don't understand the race here.
> 
> Not like SCSI, NVME doesn't complete request atomically, so double
> completion/free can be done from both timeout & nvme_cancel_request()(via teardown).
> 
> Given request is completed remotely or asynchronously in the two code paths,
> the teardown_lock can't protect the case.
> 
> One solution is to apply the introduced blk_mq_complete_request_sync()
> in both two code paths.
> 
> Another candidate is to use nvme_sync_queues() before teardown, such as
> the following change:
> 
> diff --git a/drivers/nvme/host/tcp.c b/drivers/nvme/host/tcp.c
> index d6a3e1487354..dc3561ca0074 100644
> --- a/drivers/nvme/host/tcp.c
> +++ b/drivers/nvme/host/tcp.c
> @@ -1909,6 +1909,7 @@ static void nvme_tcp_teardown_io_queues(struct nvme_ctrl *ctrl,
>   	blk_mq_quiesce_queue(ctrl->admin_q);
>   	nvme_start_freeze(ctrl);
>   	nvme_stop_queues(ctrl);
> +	nvme_sync_queues(ctrl);
nvme_sync_queues now sync io queues and admin queues, so we may need like this:
nvme_sync_io_queues(struct nvme_ctrl *ctrl)
{
	down_read(&ctrl->namespaces_rwsem);
	list_for_each_entry(ns, &ctrl->namespaces, list)
		blk_sync_queue(ns->queue);
	up_read(&ctrl->namespaces_rwsem);
}
EXPORT_SYMBOL_GPL(nvme_sync_io_queues);

void nvme_sync_queues(struct nvme_ctrl *ctrl)
{
	struct nvme_ns *ns;

     nvme_sync_io_queues(ctrl);

	if (ctrl->admin_q)
		blk_sync_queue(ctrl->admin_q);
}
EXPORT_SYMBOL_GPL(nvme_sync_queues);
>   	nvme_tcp_stop_io_queues(ctrl);
>   	if (ctrl->tagset) {
>   		blk_mq_tagset_busy_iter(ctrl->tagset,
> @@ -2171,14 +2172,11 @@ static void nvme_tcp_complete_timed_out(struct request *rq)
>   	struct nvme_tcp_request *req = blk_mq_rq_to_pdu(rq);
>   	struct nvme_ctrl *ctrl = &req->queue->ctrl->ctrl;
>   
> -	/* fence other contexts that may complete the command */
> -	mutex_lock(&to_tcp_ctrl(ctrl)->teardown_lock);
>   	nvme_tcp_stop_queue(ctrl, nvme_tcp_queue_id(req->queue));
>   	if (!blk_mq_request_completed(rq)) {
>   		nvme_req(rq)->status = NVME_SC_HOST_ABORTED_CMD;
>   		blk_mq_complete_request(rq);
>   	}
> -	mutex_unlock(&to_tcp_ctrl(ctrl)->teardown_lock);
>   }
>   
>   static enum blk_eh_timer_return
> 
> 
> Thanks,
> Ming
> 
> .
>