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

From: Bart Van Assche
Date: Wed Nov 18 2015 - 11:36:12 EST

On 11/18/2015 01:15 AM, Sagi Grimberg wrote:
On 18/11/2015 03:17, Bart Van Assche wrote:
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-> = NULL;
+ } else {
+ wr-> = &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,

Hi Bart,

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'm not so sure it actually matters if some WRs succeeded. In the normal
flow when srpt has enough available work requests (sq_wr_avail) they
should all succeed otherwise we're in trouble. If the QP transitioned
to ERROR state, then some failed, but those that succeeded will
generate flush completions, and srpt should handle it correctly
shouldn't it?

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

I'm not an expert on srpt, can you explain how this mechanism will help?

Hello Sagi,

As you know events like a cable pull can cause some of the RDMA work requests to succeed and others to fail. It is essential that all RDMA work requests related to the same SCSI command have finished before the buffers these requests operate upon are reused. The purpose of the SRPT_RDMA_ABORT request is to wait for the RDMA requests that were posted without IB_SEND_SIGNALED and for which no error completion will be received. BTW, I think this consideration applies to all SCSI target drivers and not only to SRP target drivers.

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