nfsd4: replace callback thread by asynchronous rpc

We don't really need a synchronous rpc, and moving to an asynchronous
rpc allows us to do without this extra kthread.
Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>

fs/nfsd/nfs4callback.c

@@ -415,6 +415,21 @@ static void warn_no_callback_path(struct nfs4_client *clp, int reason)
 		(int)clp->cl_name.len, clp->cl_name.data, reason);
 }
 
+static void nfsd4_cb_probe_done(struct rpc_task *task, void *calldata)
+{
+	struct nfs4_client *clp = calldata;
+
+	if (task->tk_status)
+		warn_no_callback_path(clp, task->tk_status);
+	else
+		atomic_set(&clp->cl_callback.cb_set, 1);
+	put_nfs4_client(clp);
+}
+
+static const struct rpc_call_ops nfsd4_cb_probe_ops = {
+	.rpc_call_done = nfsd4_cb_probe_done,
+};
+
 static struct rpc_cred *lookup_cb_cred(struct nfs4_callback *cb)
 {
 	struct auth_cred acred = {
@@ -431,9 +446,8 @@ static struct rpc_cred *lookup_cb_cred(struct nfs4_callback *cb)
 							RPCAUTH_LOOKUP_NEW);
 }
 
-static int do_probe_callback(void *data)
+void do_probe_callback(struct nfs4_client *clp)
 {
-	struct nfs4_client *clp = data;
 	struct nfs4_callback    *cb = &clp->cl_callback;
 	struct rpc_message msg = {
 		.rpc_proc       = &nfs4_cb_procedures[NFSPROC4_CLNT_CB_NULL],
@@ -449,15 +463,13 @@ static int do_probe_callback(void *data)
 	}
 	cb->cb_cred = cred;
 	msg.rpc_cred = cb->cb_cred;
-	status = rpc_call_sync(cb->cb_client, &msg, RPC_TASK_SOFT);
+	status = rpc_call_async(cb->cb_client, &msg, RPC_TASK_SOFT,
+				&nfsd4_cb_probe_ops, (void *)clp);
 out:
-	if (status)
+	if (status) {
 		warn_no_callback_path(clp, status);
-	else
-		atomic_set(&cb->cb_set, 1);
-
-	put_nfs4_client(clp);
-	return 0;
+		put_nfs4_client(clp);
+	}
 }
 
 /*
@@ -466,7 +478,6 @@ out:
 void
 nfsd4_probe_callback(struct nfs4_client *clp)
 {
-	struct task_struct *t;
 	int status;
 
 	BUG_ON(atomic_read(&clp->cl_callback.cb_set));
@@ -480,12 +491,7 @@ nfsd4_probe_callback(struct nfs4_client *clp)
 	/* the task holds a reference to the nfs4_client struct */
 	atomic_inc(&clp->cl_count);
 
-	t = kthread_run(do_probe_callback, clp, "nfs4_cb_probe");
-
-	if (IS_ERR(t))
-		atomic_dec(&clp->cl_count);
-
-	return;
+	do_probe_callback(clp);
 }
 
 /*