svcrdma: Preserve Receive buffer until svc_rdma_sendto

Rather than releasing the incoming svc_rdma_recv_ctxt at the end of
svc_rdma_recvfrom, hold onto it until svc_rdma_sendto.

This permits the contents of the Receive buffer to be preserved
through svc_process and then referenced directly in sendto as it
constructs Write and Reply chunks to return to the client.

The real changes will come in subsequent patches.

Note: I cannot use ->xpo_release_rqst for this purpose because that
is called _before_ ->xpo_sendto. svc_rdma_sendto uses information in
the received Call transport header to construct the Reply transport
header, which is preserved in the RPC's Receive buffer.

The historical comment in svc_send() isn't helpful: it is already
obvious that ->xpo_release_rqst is being called before ->xpo_sendto,
but there is no explanation for this ordering going back to the
beginning of the git era.
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>

net/sunrpc/xprtrdma/svc_rdma_recvfrom.c

@@ -789,7 +789,7 @@ int svc_rdma_recvfrom(struct svc_rqst *rqstp)
 		goto out_readchunk;
 
 complete:
-	svc_rdma_recv_ctxt_put(rdma_xprt, ctxt);
+	rqstp->rq_xprt_ctxt = ctxt;
 	rqstp->rq_prot = IPPROTO_MAX;
 	svc_xprt_copy_addrs(rqstp, xprt);
 	return rqstp->rq_arg.len;

net/sunrpc/xprtrdma/svc_rdma_sendto.c

@@ -623,6 +623,7 @@ int svc_rdma_sendto(struct svc_rqst *rqstp)
 	struct svc_xprt *xprt = rqstp->rq_xprt;
 	struct svcxprt_rdma *rdma =
 		container_of(xprt, struct svcxprt_rdma, sc_xprt);
+	struct svc_rdma_recv_ctxt *rctxt = rqstp->rq_xprt_ctxt;
 	__be32 *p, *rdma_argp, *rdma_resp, *wr_lst, *rp_ch;
 	struct xdr_buf *xdr = &rqstp->rq_res;
 	struct page *res_page;
@@ -675,7 +676,12 @@ int svc_rdma_sendto(struct svc_rqst *rqstp)
 				      wr_lst, rp_ch);
 	if (ret < 0)
 		goto err0;
-	return 0;
+	ret = 0;
+
+out:
+	rqstp->rq_xprt_ctxt = NULL;
+	svc_rdma_recv_ctxt_put(rdma, rctxt);
+	return ret;
 
  err2:
 	if (ret != -E2BIG && ret != -EINVAL)
@@ -684,12 +690,14 @@ int svc_rdma_sendto(struct svc_rqst *rqstp)
 	ret = svc_rdma_send_error_msg(rdma, rdma_resp, rqstp);
 	if (ret < 0)
 		goto err0;
-	return 0;
+	ret = 0;
+	goto out;
 
  err1:
 	put_page(res_page);
  err0:
 	trace_svcrdma_send_failed(rqstp, ret);
 	set_bit(XPT_CLOSE, &xprt->xpt_flags);
-	return -ENOTCONN;
+	ret = -ENOTCONN;
+	goto out;
 }