net/sunrpc/xprtrdma/backchannel.c - arm/linux - Git at Google

 /*
  * Copyright (c) 2015 Oracle.  All rights reserved.
  *
  * Support for backward direction RPCs on RPC/RDMA.
  */

 #include <linux/module.h>
 #include <linux/sunrpc/xprt.h>
 #include <linux/sunrpc/svc.h>
 #include <linux/sunrpc/svc_xprt.h>

 #include "xprt_rdma.h"

 #if IS_ENABLED(CONFIG_SUNRPC_DEBUG)
 # define RPCDBG_FACILITY	RPCDBG_TRANS
 #endif

 #undef RPCRDMA_BACKCHANNEL_DEBUG

 static void rpcrdma_bc_free_rqst(struct rpcrdma_xprt *r_xprt,
 				 struct rpc_rqst *rqst)
 {
 	struct rpcrdma_buffer *buf = &r_xprt->rx_buf;
 	struct rpcrdma_req *req = rpcr_to_rdmar(rqst);

 	spin_lock(&buf->rb_reqslock);
 	list_del(&req->rl_all);
 	spin_unlock(&buf->rb_reqslock);

 	rpcrdma_destroy_req(req);

 	kfree(rqst);
 }

 static int rpcrdma_bc_setup_rqst(struct rpcrdma_xprt *r_xprt,
 				 struct rpc_rqst *rqst)
 {
 	struct rpcrdma_regbuf *rb;
 	struct rpcrdma_req *req;
 	size_t size;

 	req = rpcrdma_create_req(r_xprt);
 	if (IS_ERR(req))
 		return PTR_ERR(req);
 	req->rl_backchannel = true;

 	rb = rpcrdma_alloc_regbuf(RPCRDMA_HDRBUF_SIZE,
 				  DMA_TO_DEVICE, GFP_KERNEL);
 	if (IS_ERR(rb))
 		goto out_fail;
 	req->rl_rdmabuf = rb;

 	size = r_xprt->rx_data.inline_rsize;
 	rb = rpcrdma_alloc_regbuf(size, DMA_TO_DEVICE, GFP_KERNEL);
 	if (IS_ERR(rb))
 		goto out_fail;
 	req->rl_sendbuf = rb;
 	xdr_buf_init(&rqst->rq_snd_buf, rb->rg_base, size);
 	rpcrdma_set_xprtdata(rqst, req);
 	return 0;

 out_fail:
 	rpcrdma_bc_free_rqst(r_xprt, rqst);
 	return -ENOMEM;
 }

 /* Allocate and add receive buffers to the rpcrdma_buffer's
  * existing list of rep's. These are released when the
  * transport is destroyed.
  */
 static int rpcrdma_bc_setup_reps(struct rpcrdma_xprt *r_xprt,
 				 unsigned int count)
 {
 	struct rpcrdma_rep *rep;
 	int rc = 0;

 	while (count--) {
 		rep = rpcrdma_create_rep(r_xprt);
 		if (IS_ERR(rep)) {
 			pr_err("RPC:       %s: reply buffer alloc failed\n",
 			       __func__);
 			rc = PTR_ERR(rep);
 			break;
 		}

 		rpcrdma_recv_buffer_put(rep);
 	}

 	return rc;
 }

 /**
  * xprt_rdma_bc_setup - Pre-allocate resources for handling backchannel requests
  * @xprt: transport associated with these backchannel resources
  * @reqs: number of concurrent incoming requests to expect
  *
  * Returns 0 on success; otherwise a negative errno
  */
 int xprt_rdma_bc_setup(struct rpc_xprt *xprt, unsigned int reqs)
 {
 	struct rpcrdma_xprt *r_xprt = rpcx_to_rdmax(xprt);
 	struct rpcrdma_buffer *buffer = &r_xprt->rx_buf;
 	struct rpc_rqst *rqst;
 	unsigned int i;
 	int rc;

 	/* The backchannel reply path returns each rpc_rqst to the
 	 * bc_pa_list _after_ the reply is sent. If the server is
 	 * faster than the client, it can send another backward
 	 * direction request before the rpc_rqst is returned to the
 	 * list. The client rejects the request in this case.
 	 *
 	 * Twice as many rpc_rqsts are prepared to ensure there is
 	 * always an rpc_rqst available as soon as a reply is sent.
 	 */
 	if (reqs > RPCRDMA_BACKWARD_WRS >> 1)
 		goto out_err;

 	for (i = 0; i < (reqs << 1); i++) {
 		rqst = kzalloc(sizeof(*rqst), GFP_KERNEL);
 		if (!rqst) {
 			pr_err("RPC:       %s: Failed to create bc rpc_rqst\n",
 			       __func__);
 			goto out_free;
 		}
 		dprintk("RPC:       %s: new rqst %p\n", __func__, rqst);

 		rqst->rq_xprt = &r_xprt->rx_xprt;
 		INIT_LIST_HEAD(&rqst->rq_list);
 		INIT_LIST_HEAD(&rqst->rq_bc_list);

 		if (rpcrdma_bc_setup_rqst(r_xprt, rqst))
 			goto out_free;

 		spin_lock_bh(&xprt->bc_pa_lock);
 		list_add(&rqst->rq_bc_pa_list, &xprt->bc_pa_list);
 		spin_unlock_bh(&xprt->bc_pa_lock);
 	}

 	rc = rpcrdma_bc_setup_reps(r_xprt, reqs);
 	if (rc)
 		goto out_free;

 	rc = rpcrdma_ep_post_extra_recv(r_xprt, reqs);
 	if (rc)
 		goto out_free;

 	buffer->rb_bc_srv_max_requests = reqs;
 	request_module("svcrdma");

 	return 0;

 out_free:
 	xprt_rdma_bc_destroy(xprt, reqs);

 out_err:
 	pr_err("RPC:       %s: setup backchannel transport failed\n", __func__);
 	return -ENOMEM;
 }

 /**
  * xprt_rdma_bc_up - Create transport endpoint for backchannel service
  * @serv: server endpoint
  * @net: network namespace
  *
  * The "xprt" is an implied argument: it supplies the name of the
  * backchannel transport class.
  *
  * Returns zero on success, negative errno on failure
  */
 int xprt_rdma_bc_up(struct svc_serv *serv, struct net *net)
 {
 	int ret;

 	ret = svc_create_xprt(serv, "rdma-bc", net, PF_INET, 0, 0);
 	if (ret < 0)
 		return ret;
 	return 0;
 }

 /**
  * xprt_rdma_bc_maxpayload - Return maximum backchannel message size
  * @xprt: transport
  *
  * Returns maximum size, in bytes, of a backchannel message
  */
 size_t xprt_rdma_bc_maxpayload(struct rpc_xprt *xprt)
 {
 	struct rpcrdma_xprt *r_xprt = rpcx_to_rdmax(xprt);
 	struct rpcrdma_create_data_internal *cdata = &r_xprt->rx_data;
 	size_t maxmsg;

 	maxmsg = min_t(unsigned int, cdata->inline_rsize, cdata->inline_wsize);
 	return maxmsg - RPCRDMA_HDRLEN_MIN;
 }

 /**
  * rpcrdma_bc_marshal_reply - Send backwards direction reply
  * @rqst: buffer containing RPC reply data
  *
  * Returns zero on success.
  */
 int rpcrdma_bc_marshal_reply(struct rpc_rqst *rqst)
 {
 	struct rpc_xprt *xprt = rqst->rq_xprt;
 	struct rpcrdma_xprt *r_xprt = rpcx_to_rdmax(xprt);
 	struct rpcrdma_req *req = rpcr_to_rdmar(rqst);
 	struct rpcrdma_msg *headerp;

 	headerp = rdmab_to_msg(req->rl_rdmabuf);
 	headerp->rm_xid = rqst->rq_xid;
 	headerp->rm_vers = rpcrdma_version;
 	headerp->rm_credit =
 			cpu_to_be32(r_xprt->rx_buf.rb_bc_srv_max_requests);
 	headerp->rm_type = rdma_msg;
 	headerp->rm_body.rm_chunks[0] = xdr_zero;
 	headerp->rm_body.rm_chunks[1] = xdr_zero;
 	headerp->rm_body.rm_chunks[2] = xdr_zero;

 	if (!rpcrdma_prepare_send_sges(&r_xprt->rx_ia, req, RPCRDMA_HDRLEN_MIN,
 				       &rqst->rq_snd_buf, rpcrdma_noch))
 		return -EIO;
 	return 0;
 }

 /**
  * xprt_rdma_bc_destroy - Release resources for handling backchannel requests
  * @xprt: transport associated with these backchannel resources
  * @reqs: number of incoming requests to destroy; ignored
  */
 void xprt_rdma_bc_destroy(struct rpc_xprt *xprt, unsigned int reqs)
 {
 	struct rpcrdma_xprt *r_xprt = rpcx_to_rdmax(xprt);
 	struct rpc_rqst *rqst, *tmp;

 	spin_lock_bh(&xprt->bc_pa_lock);
 	list_for_each_entry_safe(rqst, tmp, &xprt->bc_pa_list, rq_bc_pa_list) {
 		list_del(&rqst->rq_bc_pa_list);
 		spin_unlock_bh(&xprt->bc_pa_lock);

 		rpcrdma_bc_free_rqst(r_xprt, rqst);

 		spin_lock_bh(&xprt->bc_pa_lock);
 	}
 	spin_unlock_bh(&xprt->bc_pa_lock);
 }

 /**
  * xprt_rdma_bc_free_rqst - Release a backchannel rqst
  * @rqst: request to release
  */
 void xprt_rdma_bc_free_rqst(struct rpc_rqst *rqst)
 {
 	struct rpc_xprt *xprt = rqst->rq_xprt;

 	dprintk("RPC:       %s: freeing rqst %p (req %p)\n",
 		__func__, rqst, rpcr_to_rdmar(rqst));

 	smp_mb__before_atomic();
 	WARN_ON_ONCE(!test_bit(RPC_BC_PA_IN_USE, &rqst->rq_bc_pa_state));
 	clear_bit(RPC_BC_PA_IN_USE, &rqst->rq_bc_pa_state);
 	smp_mb__after_atomic();

 	spin_lock_bh(&xprt->bc_pa_lock);
 	list_add_tail(&rqst->rq_bc_pa_list, &xprt->bc_pa_list);
 	spin_unlock_bh(&xprt->bc_pa_lock);
 }

 /**
  * rpcrdma_bc_receive_call - Handle a backward direction call
  * @xprt: transport receiving the call
  * @rep: receive buffer containing the call
  *
  * Called in the RPC reply handler, which runs in a tasklet.
  * Be quick about it.
  *
  * Operational assumptions:
  *    o Backchannel credits are ignored, just as the NFS server
  *      forechannel currently does
  *    o The ULP manages a replay cache (eg, NFSv4.1 sessions).
  *      No replay detection is done at the transport level
  */
 void rpcrdma_bc_receive_call(struct rpcrdma_xprt *r_xprt,
 			     struct rpcrdma_rep *rep)
 {
 	struct rpc_xprt *xprt = &r_xprt->rx_xprt;
 	struct rpcrdma_msg *headerp;
 	struct svc_serv *bc_serv;
 	struct rpcrdma_req *req;
 	struct rpc_rqst *rqst;
 	struct xdr_buf *buf;
 	size_t size;
 	__be32 *p;

 	headerp = rdmab_to_msg(rep->rr_rdmabuf);
 #ifdef RPCRDMA_BACKCHANNEL_DEBUG
 	pr_info("RPC:       %s: callback XID %08x, length=%u\n",
 		__func__, be32_to_cpu(headerp->rm_xid), rep->rr_len);
 	pr_info("RPC:       %s: %*ph\n", __func__, rep->rr_len, headerp);
 #endif

 	/* Sanity check:
 	 * Need at least enough bytes for RPC/RDMA header, as code
 	 * here references the header fields by array offset. Also,
 	 * backward calls are always inline, so ensure there
 	 * are some bytes beyond the RPC/RDMA header.
 	 */
 	if (rep->rr_len < RPCRDMA_HDRLEN_MIN + 24)
 		goto out_short;
 	p = (__be32 *)((unsigned char *)headerp + RPCRDMA_HDRLEN_MIN);
 	size = rep->rr_len - RPCRDMA_HDRLEN_MIN;

 	/* Grab a free bc rqst */
 	spin_lock(&xprt->bc_pa_lock);
 	if (list_empty(&xprt->bc_pa_list)) {
 		spin_unlock(&xprt->bc_pa_lock);
 		goto out_overflow;
 	}
 	rqst = list_first_entry(&xprt->bc_pa_list,
 				struct rpc_rqst, rq_bc_pa_list);
 	list_del(&rqst->rq_bc_pa_list);
 	spin_unlock(&xprt->bc_pa_lock);
 	dprintk("RPC:       %s: using rqst %p\n", __func__, rqst);

 	/* Prepare rqst */
 	rqst->rq_reply_bytes_recvd = 0;
 	rqst->rq_bytes_sent = 0;
 	rqst->rq_xid = headerp->rm_xid;

 	rqst->rq_private_buf.len = size;
 	set_bit(RPC_BC_PA_IN_USE, &rqst->rq_bc_pa_state);

 	buf = &rqst->rq_rcv_buf;
 	memset(buf, 0, sizeof(*buf));
 	buf->head[0].iov_base = p;
 	buf->head[0].iov_len = size;
 	buf->len = size;

 	/* The receive buffer has to be hooked to the rpcrdma_req
 	 * so that it can be reposted after the server is done
 	 * parsing it but just before sending the backward
 	 * direction reply.
 	 */
 	req = rpcr_to_rdmar(rqst);
 	dprintk("RPC:       %s: attaching rep %p to req %p\n",
 		__func__, rep, req);
 	req->rl_reply = rep;

 	/* Defeat the retransmit detection logic in send_request */
 	req->rl_connect_cookie = 0;

 	/* Queue rqst for ULP's callback service */
 	bc_serv = xprt->bc_serv;
 	spin_lock(&bc_serv->sv_cb_lock);
 	list_add(&rqst->rq_bc_list, &bc_serv->sv_cb_list);
 	spin_unlock(&bc_serv->sv_cb_lock);

 	wake_up(&bc_serv->sv_cb_waitq);

 	r_xprt->rx_stats.bcall_count++;
 	return;

 out_overflow:
 	pr_warn("RPC/RDMA backchannel overflow\n");
 	xprt_disconnect_done(xprt);
 	/* This receive buffer gets reposted automatically
 	 * when the connection is re-established.
 	 */
 	return;

 out_short:
 	pr_warn("RPC/RDMA short backward direction call\n");

 	if (rpcrdma_ep_post_recv(&r_xprt->rx_ia, rep))
 		xprt_disconnect_done(xprt);
 	else
 		pr_warn("RPC:       %s: reposting rep %p\n",
 			__func__, rep);
 }
	/*
	* Copyright (c) 2015 Oracle. All rights reserved.
	*
	* Support for backward direction RPCs on RPC/RDMA.
	*/

	#include <linux/module.h>
	#include <linux/sunrpc/xprt.h>
	#include <linux/sunrpc/svc.h>
	#include <linux/sunrpc/svc_xprt.h>

	#include "xprt_rdma.h"

	#if IS_ENABLED(CONFIG_SUNRPC_DEBUG)
	# define RPCDBG_FACILITY RPCDBG_TRANS
	#endif

	#undef RPCRDMA_BACKCHANNEL_DEBUG

	static void rpcrdma_bc_free_rqst(struct rpcrdma_xprt *r_xprt,
	struct rpc_rqst *rqst)
	{
	struct rpcrdma_buffer *buf = &r_xprt->rx_buf;
	struct rpcrdma_req *req = rpcr_to_rdmar(rqst);

	spin_lock(&buf->rb_reqslock);
	list_del(&req->rl_all);
	spin_unlock(&buf->rb_reqslock);

	rpcrdma_destroy_req(req);

	kfree(rqst);
	}

	static int rpcrdma_bc_setup_rqst(struct rpcrdma_xprt *r_xprt,
	struct rpc_rqst *rqst)
	{
	struct rpcrdma_regbuf *rb;
	struct rpcrdma_req *req;
	size_t size;

	req = rpcrdma_create_req(r_xprt);
	if (IS_ERR(req))
	return PTR_ERR(req);
	req->rl_backchannel = true;

	rb = rpcrdma_alloc_regbuf(RPCRDMA_HDRBUF_SIZE,
	DMA_TO_DEVICE, GFP_KERNEL);
	if (IS_ERR(rb))
	goto out_fail;
	req->rl_rdmabuf = rb;

	size = r_xprt->rx_data.inline_rsize;
	rb = rpcrdma_alloc_regbuf(size, DMA_TO_DEVICE, GFP_KERNEL);
	if (IS_ERR(rb))
	goto out_fail;
	req->rl_sendbuf = rb;
	xdr_buf_init(&rqst->rq_snd_buf, rb->rg_base, size);
	rpcrdma_set_xprtdata(rqst, req);
	return 0;

	out_fail:
	rpcrdma_bc_free_rqst(r_xprt, rqst);
	return -ENOMEM;
	}

	/* Allocate and add receive buffers to the rpcrdma_buffer's
	* existing list of rep's. These are released when the
	* transport is destroyed.
	*/
	static int rpcrdma_bc_setup_reps(struct rpcrdma_xprt *r_xprt,
	unsigned int count)
	{
	struct rpcrdma_rep *rep;
	int rc = 0;

	while (count--) {
	rep = rpcrdma_create_rep(r_xprt);
	if (IS_ERR(rep)) {
	pr_err("RPC: %s: reply buffer alloc failed\n",
	__func__);
	rc = PTR_ERR(rep);
	break;
	}

	rpcrdma_recv_buffer_put(rep);
	}

	return rc;
	}

	/**
	* xprt_rdma_bc_setup - Pre-allocate resources for handling backchannel requests
	* @xprt: transport associated with these backchannel resources
	* @reqs: number of concurrent incoming requests to expect
	*
	* Returns 0 on success; otherwise a negative errno
	*/
	int xprt_rdma_bc_setup(struct rpc_xprt *xprt, unsigned int reqs)
	{
	struct rpcrdma_xprt *r_xprt = rpcx_to_rdmax(xprt);
	struct rpcrdma_buffer *buffer = &r_xprt->rx_buf;
	struct rpc_rqst *rqst;
	unsigned int i;
	int rc;

	/* The backchannel reply path returns each rpc_rqst to the
	* bc_pa_list _after_ the reply is sent. If the server is
	* faster than the client, it can send another backward
	* direction request before the rpc_rqst is returned to the
	* list. The client rejects the request in this case.
	*
	* Twice as many rpc_rqsts are prepared to ensure there is
	* always an rpc_rqst available as soon as a reply is sent.
	*/
	if (reqs > RPCRDMA_BACKWARD_WRS >> 1)
	goto out_err;

	for (i = 0; i < (reqs << 1); i++) {
	rqst = kzalloc(sizeof(*rqst), GFP_KERNEL);
	if (!rqst) {
	pr_err("RPC: %s: Failed to create bc rpc_rqst\n",
	__func__);
	goto out_free;
	}
	dprintk("RPC: %s: new rqst %p\n", __func__, rqst);

	rqst->rq_xprt = &r_xprt->rx_xprt;
	INIT_LIST_HEAD(&rqst->rq_list);
	INIT_LIST_HEAD(&rqst->rq_bc_list);

	if (rpcrdma_bc_setup_rqst(r_xprt, rqst))
	goto out_free;

	spin_lock_bh(&xprt->bc_pa_lock);
	list_add(&rqst->rq_bc_pa_list, &xprt->bc_pa_list);
	spin_unlock_bh(&xprt->bc_pa_lock);
	}

	rc = rpcrdma_bc_setup_reps(r_xprt, reqs);
	if (rc)
	goto out_free;

	rc = rpcrdma_ep_post_extra_recv(r_xprt, reqs);
	if (rc)
	goto out_free;

	buffer->rb_bc_srv_max_requests = reqs;
	request_module("svcrdma");

	return 0;

	out_free:
	xprt_rdma_bc_destroy(xprt, reqs);

	out_err:
	pr_err("RPC: %s: setup backchannel transport failed\n", __func__);
	return -ENOMEM;
	}

	/**
	* xprt_rdma_bc_up - Create transport endpoint for backchannel service
	* @serv: server endpoint
	* @net: network namespace
	*
	* The "xprt" is an implied argument: it supplies the name of the
	* backchannel transport class.
	*
	* Returns zero on success, negative errno on failure
	*/
	int xprt_rdma_bc_up(struct svc_serv serv, struct net net)
	{
	int ret;

	ret = svc_create_xprt(serv, "rdma-bc", net, PF_INET, 0, 0);
	if (ret < 0)
	return ret;
	return 0;
	}

	/**
	* xprt_rdma_bc_maxpayload - Return maximum backchannel message size
	* @xprt: transport
	*
	* Returns maximum size, in bytes, of a backchannel message
	*/
	size_t xprt_rdma_bc_maxpayload(struct rpc_xprt *xprt)
	{
	struct rpcrdma_xprt *r_xprt = rpcx_to_rdmax(xprt);
	struct rpcrdma_create_data_internal *cdata = &r_xprt->rx_data;
	size_t maxmsg;

	maxmsg = min_t(unsigned int, cdata->inline_rsize, cdata->inline_wsize);
	return maxmsg - RPCRDMA_HDRLEN_MIN;
	}

	/**
	* rpcrdma_bc_marshal_reply - Send backwards direction reply
	* @rqst: buffer containing RPC reply data
	*
	* Returns zero on success.
	*/
	int rpcrdma_bc_marshal_reply(struct rpc_rqst *rqst)
	{
	struct rpc_xprt *xprt = rqst->rq_xprt;
	struct rpcrdma_xprt *r_xprt = rpcx_to_rdmax(xprt);
	struct rpcrdma_req *req = rpcr_to_rdmar(rqst);
	struct rpcrdma_msg *headerp;

	headerp = rdmab_to_msg(req->rl_rdmabuf);
	headerp->rm_xid = rqst->rq_xid;
	headerp->rm_vers = rpcrdma_version;
	headerp->rm_credit =
	cpu_to_be32(r_xprt->rx_buf.rb_bc_srv_max_requests);
	headerp->rm_type = rdma_msg;
	headerp->rm_body.rm_chunks[0] = xdr_zero;
	headerp->rm_body.rm_chunks[1] = xdr_zero;
	headerp->rm_body.rm_chunks[2] = xdr_zero;

	if (!rpcrdma_prepare_send_sges(&r_xprt->rx_ia, req, RPCRDMA_HDRLEN_MIN,
	&rqst->rq_snd_buf, rpcrdma_noch))
	return -EIO;
	return 0;
	}

	/**
	* xprt_rdma_bc_destroy - Release resources for handling backchannel requests
	* @xprt: transport associated with these backchannel resources
	* @reqs: number of incoming requests to destroy; ignored
	*/
	void xprt_rdma_bc_destroy(struct rpc_xprt *xprt, unsigned int reqs)
	{
	struct rpcrdma_xprt *r_xprt = rpcx_to_rdmax(xprt);
	struct rpc_rqst rqst, tmp;

	spin_lock_bh(&xprt->bc_pa_lock);
	list_for_each_entry_safe(rqst, tmp, &xprt->bc_pa_list, rq_bc_pa_list) {
	list_del(&rqst->rq_bc_pa_list);
	spin_unlock_bh(&xprt->bc_pa_lock);

	rpcrdma_bc_free_rqst(r_xprt, rqst);

	spin_lock_bh(&xprt->bc_pa_lock);
	}
	spin_unlock_bh(&xprt->bc_pa_lock);
	}

	/**
	* xprt_rdma_bc_free_rqst - Release a backchannel rqst
	* @rqst: request to release
	*/
	void xprt_rdma_bc_free_rqst(struct rpc_rqst *rqst)
	{
	struct rpc_xprt *xprt = rqst->rq_xprt;

	dprintk("RPC: %s: freeing rqst %p (req %p)\n",
	__func__, rqst, rpcr_to_rdmar(rqst));

	smp_mb__before_atomic();
	WARN_ON_ONCE(!test_bit(RPC_BC_PA_IN_USE, &rqst->rq_bc_pa_state));
	clear_bit(RPC_BC_PA_IN_USE, &rqst->rq_bc_pa_state);
	smp_mb__after_atomic();

	spin_lock_bh(&xprt->bc_pa_lock);
	list_add_tail(&rqst->rq_bc_pa_list, &xprt->bc_pa_list);
	spin_unlock_bh(&xprt->bc_pa_lock);
	}

	/**
	* rpcrdma_bc_receive_call - Handle a backward direction call
	* @xprt: transport receiving the call
	* @rep: receive buffer containing the call
	*
	* Called in the RPC reply handler, which runs in a tasklet.
	* Be quick about it.
	*
	* Operational assumptions:
	* o Backchannel credits are ignored, just as the NFS server
	* forechannel currently does
	* o The ULP manages a replay cache (eg, NFSv4.1 sessions).
	* No replay detection is done at the transport level
	*/
	void rpcrdma_bc_receive_call(struct rpcrdma_xprt *r_xprt,
	struct rpcrdma_rep *rep)
	{
	struct rpc_xprt *xprt = &r_xprt->rx_xprt;
	struct rpcrdma_msg *headerp;
	struct svc_serv *bc_serv;
	struct rpcrdma_req *req;
	struct rpc_rqst *rqst;
	struct xdr_buf *buf;
	size_t size;
	__be32 *p;

	headerp = rdmab_to_msg(rep->rr_rdmabuf);
	#ifdef RPCRDMA_BACKCHANNEL_DEBUG
	pr_info("RPC: %s: callback XID %08x, length=%u\n",
	__func__, be32_to_cpu(headerp->rm_xid), rep->rr_len);
	pr_info("RPC: %s: %*ph\n", __func__, rep->rr_len, headerp);
	#endif

	/* Sanity check:
	* Need at least enough bytes for RPC/RDMA header, as code
	* here references the header fields by array offset. Also,
	* backward calls are always inline, so ensure there
	* are some bytes beyond the RPC/RDMA header.
	*/
	if (rep->rr_len < RPCRDMA_HDRLEN_MIN + 24)
	goto out_short;
	p = (__be32 )((unsigned char )headerp + RPCRDMA_HDRLEN_MIN);
	size = rep->rr_len - RPCRDMA_HDRLEN_MIN;

	/* Grab a free bc rqst */
	spin_lock(&xprt->bc_pa_lock);
	if (list_empty(&xprt->bc_pa_list)) {
	spin_unlock(&xprt->bc_pa_lock);
	goto out_overflow;
	}
	rqst = list_first_entry(&xprt->bc_pa_list,
	struct rpc_rqst, rq_bc_pa_list);
	list_del(&rqst->rq_bc_pa_list);
	spin_unlock(&xprt->bc_pa_lock);
	dprintk("RPC: %s: using rqst %p\n", __func__, rqst);

	/* Prepare rqst */
	rqst->rq_reply_bytes_recvd = 0;
	rqst->rq_bytes_sent = 0;
	rqst->rq_xid = headerp->rm_xid;

	rqst->rq_private_buf.len = size;
	set_bit(RPC_BC_PA_IN_USE, &rqst->rq_bc_pa_state);

	buf = &rqst->rq_rcv_buf;
	memset(buf, 0, sizeof(*buf));
	buf->head[0].iov_base = p;
	buf->head[0].iov_len = size;
	buf->len = size;

	/* The receive buffer has to be hooked to the rpcrdma_req
	* so that it can be reposted after the server is done
	* parsing it but just before sending the backward
	* direction reply.
	*/
	req = rpcr_to_rdmar(rqst);
	dprintk("RPC: %s: attaching rep %p to req %p\n",
	__func__, rep, req);
	req->rl_reply = rep;

	/* Defeat the retransmit detection logic in send_request */
	req->rl_connect_cookie = 0;

	/* Queue rqst for ULP's callback service */
	bc_serv = xprt->bc_serv;
	spin_lock(&bc_serv->sv_cb_lock);
	list_add(&rqst->rq_bc_list, &bc_serv->sv_cb_list);
	spin_unlock(&bc_serv->sv_cb_lock);

	wake_up(&bc_serv->sv_cb_waitq);

	r_xprt->rx_stats.bcall_count++;
	return;

	out_overflow:
	pr_warn("RPC/RDMA backchannel overflow\n");
	xprt_disconnect_done(xprt);
	/* This receive buffer gets reposted automatically
	* when the connection is re-established.
	*/
	return;

	out_short:
	pr_warn("RPC/RDMA short backward direction call\n");

	if (rpcrdma_ep_post_recv(&r_xprt->rx_ia, rep))
	xprt_disconnect_done(xprt);
	else
	pr_warn("RPC: %s: reposting rep %p\n",
	__func__, rep);
	}