svcrdma: Select NFSv4.1 backchannel transport based on forward channel

The current code always selects XPRT_TRANSPORT_BC_TCP for the back
channel, even when the forward channel was not TCP (eg, RDMA). When
a 4.1 mount is attempted with RDMA, the server panics in the TCP BC
code when trying to send CB_NULL.

Instead, construct the transport protocol number from the forward
channel transport or'd with XPRT_TRANSPORT_BC. Transports that do
not support bi-directional RPC will not have registered a "BC"
transport, causing create_backchannel_client() to fail immediately.

Fixes: https://bugzilla.linux-nfs.org/show_bug.cgi?id=265
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>

fs/nfsd/nfs4callback.c

@@ -689,7 +689,8 @@ static int setup_callback_client(struct nfs4_client *clp, struct nfs4_cb_conn *c
 		clp->cl_cb_session = ses;
 		args.bc_xprt = conn->cb_xprt;
 		args.prognumber = clp->cl_cb_session->se_cb_prog;
-		args.protocol = XPRT_TRANSPORT_BC_TCP;
+		args.protocol = conn->cb_xprt->xpt_class->xcl_ident |
+				XPRT_TRANSPORT_BC;
 		args.authflavor = ses->se_cb_sec.flavor;
 	}
 	/* Create RPC client */

include/linux/sunrpc/svc_xprt.h

@@ -33,6 +33,7 @@ struct svc_xprt_class {
 	struct svc_xprt_ops	*xcl_ops;
 	struct list_head	xcl_list;
 	u32			xcl_max_payload;
+	int			xcl_ident;
 };
 
 /*

net/sunrpc/svcsock.c

@@ -692,6 +692,7 @@ static struct svc_xprt_class svc_udp_class = {
 	.xcl_owner = THIS_MODULE,
 	.xcl_ops = &svc_udp_ops,
 	.xcl_max_payload = RPCSVC_MAXPAYLOAD_UDP,
+	.xcl_ident = XPRT_TRANSPORT_UDP,
 };
 
 static void svc_udp_init(struct svc_sock *svsk, struct svc_serv *serv)
@@ -1292,6 +1293,7 @@ static struct svc_xprt_class svc_tcp_class = {
 	.xcl_owner = THIS_MODULE,
 	.xcl_ops = &svc_tcp_ops,
 	.xcl_max_payload = RPCSVC_MAXPAYLOAD_TCP,
+	.xcl_ident = XPRT_TRANSPORT_TCP,
 };
 
 void svc_init_xprt_sock(void)

net/sunrpc/xprt.c

@@ -1306,7 +1306,7 @@ struct rpc_xprt *xprt_create_transport(struct xprt_create *args)
 		}
 	}
 	spin_unlock(&xprt_list_lock);
-	printk(KERN_ERR "RPC: transport (%d) not supported\n", args->ident);
+	dprintk("RPC: transport (%d) not supported\n", args->ident);
 	return ERR_PTR(-EIO);
 
 found:

net/sunrpc/xprtrdma/svc_rdma_transport.c

@@ -92,6 +92,7 @@ struct svc_xprt_class svc_rdma_class = {
 	.xcl_owner = THIS_MODULE,
 	.xcl_ops = &svc_rdma_ops,
 	.xcl_max_payload = RPCSVC_MAXPAYLOAD_TCP,
+	.xcl_ident = XPRT_TRANSPORT_RDMA,
 };
 
 struct svc_rdma_op_ctxt *svc_rdma_get_context(struct svcxprt_rdma *xprt)