NFSD: Add cb_lost tracepoint

Provide more clarity about when the callback channel is in trouble.
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>

fs/nfsd/nfs4state.c

@@ -1745,6 +1745,8 @@ static void nfsd4_conn_lost(struct svc_xpt_user *u)
 	struct nfsd4_conn *c = container_of(u, struct nfsd4_conn, cn_xpt_user);
 	struct nfs4_client *clp = c->cn_session->se_client;
 
+	trace_nfsd_cb_lost(clp);
+
 	spin_lock(&clp->cl_lock);
 	if (!list_empty(&c->cn_persession)) {
 		list_del(&c->cn_persession);

fs/nfsd/trace.h

@@ -912,6 +912,7 @@ DEFINE_EVENT(nfsd_cb_class, nfsd_cb_##name,		\
 
 DEFINE_NFSD_CB_EVENT(setup);
 DEFINE_NFSD_CB_EVENT(state);
+DEFINE_NFSD_CB_EVENT(lost);
 DEFINE_NFSD_CB_EVENT(shutdown);
 
 TRACE_DEFINE_ENUM(RPC_AUTH_NULL);