knfsd: spawn kernel thread to probe callback channel

We want to allow gss on the callback channel, so people using krb5 can
still get the benefits of delegations.

But looking up the rpc credential can take some time in that case.  And
we shouldn't delay the response to setclientid_confirm while we wait.

It may be inefficient, but for now the simplest solution is just to
spawn a new thread as necessary for the purpose.

(Thanks to Adrian Bunk for catching a missing static here.)
Signed-off-by: N"J. Bruce Fields" <bfields@citi.umich.edu>
Cc: Adrian Bunk <bunk@kernel.org>

fs/nfsd/nfs4callback.c

@@ -39,6 +39,7 @@
 #include <linux/errno.h>
 #include <linux/delay.h>
 #include <linux/sched.h>
+#include <linux/kthread.h>
 #include <linux/sunrpc/xdr.h>
 #include <linux/sunrpc/svc.h>
 #include <linux/sunrpc/clnt.h>
@@ -365,6 +366,35 @@ nfsd4_lookupcred(struct nfs4_client *clp, int taskflags)
         return ret;
 }
 
+/* Reference counting, callback cleanup, etc., all look racy as heck.
+ * And why is cb_set an atomic? */
+
+static int do_probe_callback(void *data)
+{
+	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],
+		.rpc_argp       = clp,
+	};
+	int status;
+
+	msg.rpc_cred = nfsd4_lookupcred(clp, 0);
+	if (IS_ERR(msg.rpc_cred))
+		goto out;
+	status = rpc_call_sync(cb->cb_client, &msg, RPC_TASK_SOFT);
+	put_rpccred(msg.rpc_cred);
+
+	if (status) {
+		rpc_shutdown_client(cb->cb_client);
+		cb->cb_client = NULL;
+	} else
+		atomic_set(&cb->cb_set, 1);
+out:
+	put_nfs4_client(clp);
+	return 0;
+}
+
 /*
  * Set up the callback client and put a NFSPROC4_CB_NULL on the wire...
  */
@@ -390,11 +420,7 @@ nfsd4_probe_callback(struct nfs4_client *clp)
 		.authflavor	= RPC_AUTH_UNIX,	/* XXX: need AUTH_GSS... */
 		.flags		= (RPC_CLNT_CREATE_NOPING),
 	};
-	struct rpc_message msg = {
-		.rpc_proc       = &nfs4_cb_procedures[NFSPROC4_CLNT_CB_NULL],
-		.rpc_argp       = clp,
-	};
-	int status;
+	struct task_struct *t;
 
 	if (atomic_read(&cb->cb_set))
 		return;
@@ -426,16 +452,11 @@ nfsd4_probe_callback(struct nfs4_client *clp)
 	/* the task holds a reference to the nfs4_client struct */
 	atomic_inc(&clp->cl_count);
 
-	msg.rpc_cred = nfsd4_lookupcred(clp,0);
-	if (IS_ERR(msg.rpc_cred))
-		goto out_release_clp;
-	status = rpc_call_async(cb->cb_client, &msg, RPC_TASK_ASYNC, &nfs4_cb_null_ops, NULL);
-	put_rpccred(msg.rpc_cred);
+	t = kthread_run(do_probe_callback, clp, "nfs4_cb_probe");
 
-	if (status != 0) {
-		dprintk("NFSD: asynchronous NFSPROC4_CB_NULL failed!\n");
+	if (IS_ERR(t))
 		goto out_release_clp;
-	}
+
 	return;
 
 out_release_clp:
@@ -447,30 +468,6 @@ nfsd4_probe_callback(struct nfs4_client *clp)
 		(int)clp->cl_name.len, clp->cl_name.data);
 }
 
-static void
-nfs4_cb_null(struct rpc_task *task, void *dummy)
-{
-	struct nfs4_client *clp = (struct nfs4_client *)task->tk_msg.rpc_argp;
-	struct nfs4_callback *cb = &clp->cl_callback;
-	__be32 addr = htonl(cb->cb_addr);
-
-	dprintk("NFSD: nfs4_cb_null task->tk_status %d\n", task->tk_status);
-
-	if (task->tk_status < 0) {
-		dprintk("NFSD: callback establishment to client %.*s failed\n",
-			(int)clp->cl_name.len, clp->cl_name.data);
-		goto out;
-	}
-	atomic_set(&cb->cb_set, 1);
-	dprintk("NFSD: callback set to client %u.%u.%u.%u\n", NIPQUAD(addr));
-out:
-	put_nfs4_client(clp);
-}
-
-static const struct rpc_call_ops nfs4_cb_null_ops = {
-	.rpc_call_done = nfs4_cb_null,
-};
-
 /*
  * called with dp->dl_count inc'ed.
  * nfs4_lock_state() may or may not have been called.