NFSD: Clean up nfsd4_encode_replay()

Replace open-coded encoding logic with the use of conventional XDR
utility functions. Add a tracepoint to make replays observable in
field troubleshooting situations.

The WARN_ON is removed. A stack trace is of little use, as there is
only one call site for nfsd4_encode_replay(), and a buffer length
shortage here is unlikely.
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

fs/nfsd/nfs4xdr.c

@@ -5732,27 +5732,24 @@ nfsd4_encode_operation(struct nfsd4_compoundres *resp, struct nfsd4_op *op)
 	rqstp->rq_next_page = xdr->page_ptr + 1;
 }
 
-/* 
- * Encode the reply stored in the stateowner reply cache 
+/**
+ * nfsd4_encode_replay - encode a result stored in the stateowner reply cache
+ * @xdr: send buffer's XDR stream
+ * @op: operation being replayed
  *
- * XDR note: do not encode rp->rp_buflen: the buffer contains the
- * previously sent already encoded operation.
+ * @op->replay->rp_buf contains the previously-sent already-encoded result.
  */
-void
-nfsd4_encode_replay(struct xdr_stream *xdr, struct nfsd4_op *op)
+void nfsd4_encode_replay(struct xdr_stream *xdr, struct nfsd4_op *op)
 {
-	__be32 *p;
 	struct nfs4_replay *rp = op->replay;
 
-	p = xdr_reserve_space(xdr, 8 + rp->rp_buflen);
-	if (!p) {
-		WARN_ON_ONCE(1);
-		return;
-	}
-	*p++ = cpu_to_be32(op->opnum);
-	*p++ = rp->rp_status;  /* already xdr'ed */
+	trace_nfsd_stateowner_replay(op->opnum, rp);
 
-	p = xdr_encode_opaque_fixed(p, rp->rp_buf, rp->rp_buflen);
+	if (xdr_stream_encode_u32(xdr, op->opnum) != XDR_UNIT)
+		return;
+	if (xdr_stream_encode_be32(xdr, rp->rp_status) != XDR_UNIT)
+		return;
+	xdr_stream_encode_opaque_fixed(xdr, rp->rp_buf, rp->rp_buflen);
 }
 
 void nfsd4_release_compoundargs(struct svc_rqst *rqstp)

fs/nfsd/trace.h

@@ -696,6 +696,24 @@ DEFINE_EVENT(nfsd_stid_class, nfsd_stid_##name,			\
 
 DEFINE_STID_EVENT(revoke);
 
+TRACE_EVENT(nfsd_stateowner_replay,
+	TP_PROTO(
+		u32 opnum,
+		const struct nfs4_replay *rp
+	),
+	TP_ARGS(opnum, rp),
+	TP_STRUCT__entry(
+		__field(unsigned long, status)
+		__field(u32, opnum)
+	),
+	TP_fast_assign(
+		__entry->status = be32_to_cpu(rp->rp_status);
+		__entry->opnum = opnum;
+	),
+	TP_printk("opnum=%u status=%lu",
+		__entry->opnum, __entry->status)
+);
+
 TRACE_EVENT_CONDITION(nfsd_seq4_status,
 	TP_PROTO(
 		const struct svc_rqst *rqstp,