svcrdma: Eliminate allocation of recv_ctxt objects in backchannel

The svc_rdma_recv_ctxt free list uses a lockless list to avoid the
need for a spin lock in the fast path. llist_del_first(), which is
used by svc_rdma_recv_ctxt_get(), requires serialization, however,
when there are multiple list producers that are unserialized.

I mistakenly thought there was only one caller of
svc_rdma_recv_ctxt_get() (svc_rdma_refresh_recvs()), thus explicit
serialization would not be necessary. But there is another caller:
svc_rdma_bc_sendto(), and these two are not serialized against each
other. I haven't seen ill effects that I could directly ascribe to
a lack of serialization. It's just an observation based on code
audit.

When DMA-mapping before sending a Reply, the passed-in struct
svc_rdma_recv_ctxt is used only for its write and reply PCLs. These
are currently always empty in the backchannel case. So, instead of
passing a full svc_rdma_recv_ctxt object to
svc_rdma_map_reply_msg(), let's pass in just the Write and Reply
PCLs.

This change makes it unnecessary for the backchannel to acquire a
dummy svc_rdma_recv_ctxt object when sending an RPC Call. The need
for svc_rdma_recv_ctxt free list serialization is now completely
avoided.
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

include/linux/sunrpc/svc_rdma.h

@@ -200,7 +200,8 @@ extern int svc_rdma_send(struct svcxprt_rdma *rdma,
 			 struct svc_rdma_send_ctxt *ctxt);
 extern int svc_rdma_map_reply_msg(struct svcxprt_rdma *rdma,
 				  struct svc_rdma_send_ctxt *sctxt,
-				  const struct svc_rdma_recv_ctxt *rctxt,
+				  const struct svc_rdma_pcl *write_pcl,
+				  const struct svc_rdma_pcl *reply_pcl,
 				  const struct xdr_buf *xdr);
 extern void svc_rdma_send_error_msg(struct svcxprt_rdma *rdma,
 				    struct svc_rdma_send_ctxt *sctxt,

net/sunrpc/xprtrdma/svc_rdma_backchannel.c

@@ -76,15 +76,12 @@ static int svc_rdma_bc_sendto(struct svcxprt_rdma *rdma,
 			      struct rpc_rqst *rqst,
 			      struct svc_rdma_send_ctxt *sctxt)
 {
-	struct svc_rdma_recv_ctxt *rctxt;
+	struct svc_rdma_pcl empty_pcl;
 	int ret;
 
-	rctxt = svc_rdma_recv_ctxt_get(rdma);
-	if (!rctxt)
-		return -EIO;
-
-	ret = svc_rdma_map_reply_msg(rdma, sctxt, rctxt, &rqst->rq_snd_buf);
-	svc_rdma_recv_ctxt_put(rdma, rctxt);
+	pcl_init(&empty_pcl);
+	ret = svc_rdma_map_reply_msg(rdma, sctxt, &empty_pcl, &empty_pcl,
+				     &rqst->rq_snd_buf);
 	if (ret < 0)
 		return -EIO;
 

net/sunrpc/xprtrdma/svc_rdma_sendto.c

@@ -653,7 +653,7 @@ static int svc_rdma_xb_count_sges(const struct xdr_buf *xdr,
  * svc_rdma_pull_up_needed - Determine whether to use pull-up
  * @rdma: controlling transport
  * @sctxt: send_ctxt for the Send WR
- * @rctxt: Write and Reply chunks provided by client
+ * @write_pcl: Write chunk list provided by client
  * @xdr: xdr_buf containing RPC message to transmit
  *
  * Returns:
@@ -662,7 +662,7 @@ static int svc_rdma_xb_count_sges(const struct xdr_buf *xdr,
  */
 static bool svc_rdma_pull_up_needed(const struct svcxprt_rdma *rdma,
 				    const struct svc_rdma_send_ctxt *sctxt,
-				    const struct svc_rdma_recv_ctxt *rctxt,
+				    const struct svc_rdma_pcl *write_pcl,
 				    const struct xdr_buf *xdr)
 {
 	/* Resources needed for the transport header */
@@ -672,7 +672,7 @@ static bool svc_rdma_pull_up_needed(const struct svcxprt_rdma *rdma,
 	};
 	int ret;
 
-	ret = pcl_process_nonpayloads(&rctxt->rc_write_pcl, xdr,
+	ret = pcl_process_nonpayloads(write_pcl, xdr,
 				      svc_rdma_xb_count_sges, &args);
 	if (ret < 0)
 		return false;
@@ -728,7 +728,7 @@ static int svc_rdma_xb_linearize(const struct xdr_buf *xdr,
  * svc_rdma_pull_up_reply_msg - Copy Reply into a single buffer
  * @rdma: controlling transport
  * @sctxt: send_ctxt for the Send WR; xprt hdr is already prepared
- * @rctxt: Write and Reply chunks provided by client
+ * @write_pcl: Write chunk list provided by client
  * @xdr: prepared xdr_buf containing RPC message
  *
  * The device is not capable of sending the reply directly.
@@ -743,7 +743,7 @@ static int svc_rdma_xb_linearize(const struct xdr_buf *xdr,
  */
 static int svc_rdma_pull_up_reply_msg(const struct svcxprt_rdma *rdma,
 				      struct svc_rdma_send_ctxt *sctxt,
-				      const struct svc_rdma_recv_ctxt *rctxt,
+				      const struct svc_rdma_pcl *write_pcl,
 				      const struct xdr_buf *xdr)
 {
 	struct svc_rdma_pullup_data args = {
@@ -751,7 +751,7 @@ static int svc_rdma_pull_up_reply_msg(const struct svcxprt_rdma *rdma,
 	};
 	int ret;
 
-	ret = pcl_process_nonpayloads(&rctxt->rc_write_pcl, xdr,
+	ret = pcl_process_nonpayloads(write_pcl, xdr,
 				      svc_rdma_xb_linearize, &args);
 	if (ret < 0)
 		return ret;
@@ -764,7 +764,8 @@ static int svc_rdma_pull_up_reply_msg(const struct svcxprt_rdma *rdma,
 /* svc_rdma_map_reply_msg - DMA map the buffer holding RPC message
  * @rdma: controlling transport
  * @sctxt: send_ctxt for the Send WR
- * @rctxt: Write and Reply chunks provided by client
+ * @write_pcl: Write chunk list provided by client
+ * @reply_pcl: Reply chunk provided by client
  * @xdr: prepared xdr_buf containing RPC message
  *
  * Returns:
@@ -776,7 +777,8 @@ static int svc_rdma_pull_up_reply_msg(const struct svcxprt_rdma *rdma,
  */
 int svc_rdma_map_reply_msg(struct svcxprt_rdma *rdma,
 			   struct svc_rdma_send_ctxt *sctxt,
-			   const struct svc_rdma_recv_ctxt *rctxt,
+			   const struct svc_rdma_pcl *write_pcl,
+			   const struct svc_rdma_pcl *reply_pcl,
 			   const struct xdr_buf *xdr)
 {
 	struct svc_rdma_map_data args = {
@@ -789,18 +791,18 @@ int svc_rdma_map_reply_msg(struct svcxprt_rdma *rdma,
 	sctxt->sc_sges[0].length = sctxt->sc_hdrbuf.len;
 
 	/* If there is a Reply chunk, nothing follows the transport
-	 * header, and we're done here.
+	 * header, so there is nothing to map.
 	 */
-	if (!pcl_is_empty(&rctxt->rc_reply_pcl))
+	if (!pcl_is_empty(reply_pcl))
 		return 0;
 
 	/* For pull-up, svc_rdma_send() will sync the transport header.
 	 * No additional DMA mapping is necessary.
 	 */
-	if (svc_rdma_pull_up_needed(rdma, sctxt, rctxt, xdr))
-		return svc_rdma_pull_up_reply_msg(rdma, sctxt, rctxt, xdr);
+	if (svc_rdma_pull_up_needed(rdma, sctxt, write_pcl, xdr))
+		return svc_rdma_pull_up_reply_msg(rdma, sctxt, write_pcl, xdr);
 
-	return pcl_process_nonpayloads(&rctxt->rc_write_pcl, xdr,
+	return pcl_process_nonpayloads(write_pcl, xdr,
 				       svc_rdma_xb_dma_map, &args);
 }
 
@@ -848,7 +850,8 @@ static int svc_rdma_send_reply_msg(struct svcxprt_rdma *rdma,
 {
 	int ret;
 
-	ret = svc_rdma_map_reply_msg(rdma, sctxt, rctxt, &rqstp->rq_res);
+	ret = svc_rdma_map_reply_msg(rdma, sctxt, &rctxt->rc_write_pcl,
+				     &rctxt->rc_reply_pcl, &rqstp->rq_res);
 	if (ret < 0)
 		return ret;