NFSv4: add cl_root_cred for use when machine cred is not available.

NFSv4 state management tries a root credential when no machine
credential is available, as can happen with kerberos.
It does this by replacing the cl_machine_cred with a root credential.
This means that any user of the machine credential needs to take
a lock while getting a reference to the machine credential, which is
a little cumbersome.

So introduce an explicit cl_root_cred, and never free either
credential until client shutdown.  This means that no locking
is needed to reference these credentials.  Future patches
will make use of this.

This is only a temporary addition.  both cl_machine_cred and
cl_root_cred will disappear later in the series.

Signed-off-by: NeilBrown <neilb@suse.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>

fs/nfs/client.c

@@ -248,6 +248,8 @@ void nfs_free_client(struct nfs_client *clp)
 
 	if (clp->cl_machine_cred != NULL)
 		put_rpccred(clp->cl_machine_cred);
+	if (clp->cl_root_cred != NULL)
+		put_rpccred(clp->cl_root_cred);
 
 	put_net(clp->cl_net);
 	put_nfs_version(clp->cl_nfs_mod);

fs/nfs/nfs4state.c

@@ -166,24 +166,28 @@ out:
 
 struct rpc_cred *nfs4_get_machine_cred_locked(struct nfs_client *clp)
 {
-	struct rpc_cred *cred = NULL;
+	struct rpc_cred *cred = clp->cl_root_cred;
 
-	if (clp->cl_machine_cred != NULL)
-		cred = get_rpccred(clp->cl_machine_cred);
+	if (!cred)
+		cred = clp->cl_machine_cred;
+	if (cred)
+		return get_rpccred(cred);
 	return cred;
 }
 
 static void nfs4_root_machine_cred(struct nfs_client *clp)
 {
-	struct rpc_cred *cred, *new;
+	struct rpc_cred *new;
 
 	new = rpc_lookup_machine_cred(NULL);
 	spin_lock(&clp->cl_lock);
-	cred = clp->cl_machine_cred;
-	clp->cl_machine_cred = new;
+	if (clp->cl_root_cred == NULL) {
+		clp->cl_root_cred = new;
+		new = NULL;
+	}
 	spin_unlock(&clp->cl_lock);
-	if (cred != NULL)
-		put_rpccred(cred);
+	if (new != NULL)
+		put_rpccred(new);
 }
 
 static struct rpc_cred *

include/linux/nfs_fs_sb.h

@@ -59,6 +59,7 @@ struct nfs_client {
 
 	u32			cl_minorversion;/* NFSv4 minorversion */
 	struct rpc_cred		*cl_machine_cred;
+	struct rpc_cred		*cl_root_cred;	/* Use when machine_cred is ineffective */
 
 #if IS_ENABLED(CONFIG_NFS_V4)
 	struct list_head	cl_ds_clients; /* auth flavor data servers */