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>

1 files changed, 2 insertions, 1 deletions

diff --git a/fs/nfsd/nfs4callback.c b/fs/nfsd/nfs4callback.c
index a88a93e09d69..564d72304613 100644
--- a/fs/nfsd/nfs4callback.c
+++ b/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 */