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To:     Daniel Wagner <dwagner@suse.de>, linux-nvme@lists.infradead.org
Cc:     linux-kernel@vger.kernel.org,
        James Smart <james.smart@broadcom.com>,
        Keith Busch <kbusch@kernel.org>,
        Ming Lei <ming.lei@redhat.com>,
        Sagi Grimberg <sagi@grimberg.me>,
        Wen Xiong <wenxiong@us.ibm.com>,
        James Smart <jsmart2021@gmail.com>
References: <20210802112658.75875-1-dwagner@suse.de>
 <20210802112658.75875-7-dwagner@suse.de>
From:   Hannes Reinecke <hare@suse.de>
Subject: Re: [PATCH v4 6/8] nvme-fc: fix controller reset hang during traffic
Message-ID: <79c89923-f586-79e7-6dfd-c15ceb21f569@suse.de>
Date:   Wed, 4 Aug 2021 09:23:49 +0200
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On 8/2/21 1:26 PM, Daniel Wagner wrote:
> From: James Smart <jsmart2021@gmail.com>
> 
> commit fe35ec58f0d3 ("block: update hctx map when use multiple maps")
> exposed an issue where we may hang trying to wait for queue freeze
> during I/O. We call blk_mq_update_nr_hw_queues which may attempt to freeze
> the queue. However we never started queue freeze when starting the
> reset, which means that we have inflight pending requests that entered the
> queue that we will not complete once the queue is quiesced.
> 
> So start a freeze before we quiesce the queue, and unfreeze the queue
> after we successfully connected the I/O queues (the unfreeze is already
> present in the code). blk_mq_update_nr_hw_queues will be called only
> after we are sure that the queue was already frozen.
> 
> This follows to how the pci driver handles resets.
> 
> This patch added logic introduced in commit 9f98772ba307 "nvme-rdma: fix
> controller reset hang during traffic".
> 
> Signed-off-by: James Smart <jsmart2021@gmail.com>
> CC: Sagi Grimberg <sagi@grimberg.me>
> [dwagner: call nvme_unfreeze() unconditionally in
>           nvme_fc_recreate_io_queues() to match the nvme_start_freeze()]
> Tested-by: Daniel Wagner <dwagner@suse.de>
> Reviewed-by: Daniel Wagner <dwagner@suse.de>
> ---
>  drivers/nvme/host/fc.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/nvme/host/fc.c b/drivers/nvme/host/fc.c
> index 133b87db4f1d..b292af0fd655 100644
> --- a/drivers/nvme/host/fc.c
> +++ b/drivers/nvme/host/fc.c
> @@ -2486,6 +2486,7 @@ __nvme_fc_abort_outstanding_ios(struct nvme_fc_ctrl *ctrl, bool start_queues)
>  	 * (but with error status).
>  	 */
>  	if (ctrl->ctrl.queue_count > 1) {
> +		nvme_start_freeze(&ctrl->ctrl);
>  		nvme_stop_queues(&ctrl->ctrl);
>  		nvme_sync_io_queues(&ctrl->ctrl);
>  		blk_mq_tagset_busy_iter(&ctrl->tag_set,
> @@ -2966,8 +2967,8 @@ nvme_fc_recreate_io_queues(struct nvme_fc_ctrl *ctrl)
>  			return -ENODEV;
>  		}
>  		blk_mq_update_nr_hw_queues(&ctrl->tag_set, nr_io_queues);
> -		nvme_unfreeze(&ctrl->ctrl);
>  	}
> +	nvme_unfreeze(&ctrl->ctrl);
>  
>  	ret = nvme_fc_create_hw_io_queues(ctrl, ctrl->ctrl.sqsize + 1);
>  	if (ret)
> 
There still is now an imbalance, as we're always calling
'nvme_unfreeze()' (irrespective on the number of queues), but will only
call 'nvme_start_freeze()' if we have more than one queue.

This might lead to an imbalance on the mq_freeze_depth counter.
Wouldn't it be better to move the call to 'nvme_start_freeze()' out of
the if() condition to avoid the imbalance?

Cheers,

Hannes
-- 
Dr. Hannes Reinecke		           Kernel Storage Architect
hare@suse.de			                  +49 911 74053 688
SUSE Software Solutions Germany GmbH, Maxfeldstr. 5, 90409 Nürnberg
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