NFS: Allow walking nfs_client.cl_superblocks list outside client.c

We're about to move some fields from struct nfs_client to struct
nfs_server.  There is a many-to-one relationship between nfs_servers
and nfs_clients.  After these fields are moved to the nfs_server
struct, to visit all of the data in these fields that is owned by one
nfs_client, code will need to visit each nfs_server on the
cl_superblocks list for that nfs_client.

To serialize changes to the cl_superblocks list during these little
expeditions, protect the list with RCU.
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>

fs/nfs/client.c

@@ -1003,6 +1003,27 @@ static void nfs_server_copy_userdata(struct nfs_server *target, struct nfs_serve
 	target->options = source->options;
 }
 
+static void nfs_server_insert_lists(struct nfs_server *server)
+{
+	struct nfs_client *clp = server->nfs_client;
+
+	spin_lock(&nfs_client_lock);
+	list_add_tail_rcu(&server->client_link, &clp->cl_superblocks);
+	list_add_tail(&server->master_link, &nfs_volume_list);
+	spin_unlock(&nfs_client_lock);
+
+}
+
+static void nfs_server_remove_lists(struct nfs_server *server)
+{
+	spin_lock(&nfs_client_lock);
+	list_del_rcu(&server->client_link);
+	list_del(&server->master_link);
+	spin_unlock(&nfs_client_lock);
+
+	synchronize_rcu();
+}
+
 /*
  * Allocate and initialise a server record
  */
@@ -1046,11 +1067,8 @@ void nfs_free_server(struct nfs_server *server)
 {
 	dprintk("--> nfs_free_server()\n");
 
+	nfs_server_remove_lists(server);
 	unset_pnfs_layoutdriver(server);
-	spin_lock(&nfs_client_lock);
-	list_del(&server->client_link);
-	list_del(&server->master_link);
-	spin_unlock(&nfs_client_lock);
 
 	if (server->destroy != NULL)
 		server->destroy(server);
@@ -1125,11 +1143,7 @@ struct nfs_server *nfs_create_server(const struct nfs_parsed_mount_data *data,
 		(unsigned long long) server->fsid.major,
 		(unsigned long long) server->fsid.minor);
 
-	spin_lock(&nfs_client_lock);
-	list_add_tail(&server->client_link, &server->nfs_client->cl_superblocks);
-	list_add_tail(&server->master_link, &nfs_volume_list);
-	spin_unlock(&nfs_client_lock);
-
+	nfs_server_insert_lists(server);
 	server->mount_time = jiffies;
 	nfs_free_fattr(fattr);
 	return server;
@@ -1454,11 +1468,7 @@ static int nfs4_server_common_setup(struct nfs_server *server,
 	if (server->namelen == 0 || server->namelen > NFS4_MAXNAMLEN)
 		server->namelen = NFS4_MAXNAMLEN;
 
-	spin_lock(&nfs_client_lock);
-	list_add_tail(&server->client_link, &server->nfs_client->cl_superblocks);
-	list_add_tail(&server->master_link, &nfs_volume_list);
-	spin_unlock(&nfs_client_lock);
-
+	nfs_server_insert_lists(server);
 	server->mount_time = jiffies;
 out:
 	nfs_free_fattr(fattr);
@@ -1663,11 +1673,7 @@ struct nfs_server *nfs_clone_server(struct nfs_server *source,
 	if (error < 0)
 		goto out_free_server;
 
-	spin_lock(&nfs_client_lock);
-	list_add_tail(&server->client_link, &server->nfs_client->cl_superblocks);
-	list_add_tail(&server->master_link, &nfs_volume_list);
-	spin_unlock(&nfs_client_lock);
-
+	nfs_server_insert_lists(server);
 	server->mount_time = jiffies;
 
 	nfs_free_fattr(fattr_fsinfo);

fs/nfs/nfs4renewd.c

@@ -63,9 +63,14 @@ nfs4_renew_state(struct work_struct *work)
 
 	ops = clp->cl_mvops->state_renewal_ops;
 	dprintk("%s: start\n", __func__);
-	/* Are there any active superblocks? */
-	if (list_empty(&clp->cl_superblocks))
+
+	rcu_read_lock();
+	if (list_empty(&clp->cl_superblocks)) {
+		rcu_read_unlock();
 		goto out;
+	}
+	rcu_read_unlock();
+
 	spin_lock(&clp->cl_lock);
 	lease = clp->cl_lease_time;
 	last = clp->cl_last_renewal;