linux-kernel - Re: [PATCH 4/9] srpt: chain RDMA READ/WRITE requests

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Date:	Tue, 17 Nov 2015 17:17:35 -0800
From:	Bart Van Assche <bart.vanassche@...disk.com>
To:	Christoph Hellwig <hch@....de>, <linux-rdma@...r.kernel.org>
CC:	<sagig@....mellanox.co.il>, <bart.vanassche@...disk.com>,
	<axboe@...com>, <linux-scsi@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 4/9] srpt: chain RDMA READ/WRITE requests

On 11/13/2015 05:46 AM, Christoph Hellwig wrote:
> -		ret = ib_post_send(ch->qp, &wr.wr, &bad_wr);
> -		if (ret)
> -			break;
> +		if (i == n_rdma - 1) {
> +			/* only get completion event for the last rdma read */
> +			if (dir == DMA_TO_DEVICE)
> +				wr->wr.send_flags = IB_SEND_SIGNALED;
> +			wr->wr.next = NULL;
> +		} else {
> +			wr->wr.next = &ioctx->rdma_ius[i + 1].wr;
> +		}
>   	}
>
> +	ret = ib_post_send(ch->qp, &ioctx->rdma_ius->wr, &bad_wr);
>   	if (ret)
>   		pr_err("%s[%d]: ib_post_send() returned %d for %d/%d\n",
>   				 __func__, __LINE__, ret, i, n_rdma);

Hello Christoph,

Chaining RDMA requests is a great idea. But it seems to me that this 
patch is based on the assumption that posting multiple RDMA requests 
either succeeds as a whole or fails as a whole. Sorry but I'm not sure 
that the verbs API guarantees this. In the ib_srpt driver a QP can be 
changed at any time into the error state and there might be drivers that 
report an immediate failure in that case. I think even when chaining 
RDMA requests that we still need a mechanism to wait until ongoing RDMA 
transfers have finished if some but not all RDMA requests have been posted.

Bart.
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