btrfs: make btrfs_root::free_objectid hold the next available objectid

Adjust the way free_objectid is being initialized, it now stores
BTRFS_FIRST_FREE_OBJECTID rather than the, somewhat arbitrary,
BTRFS_FIRST_FREE_OBJECTID - 1. This change also has the added benefit
that now it becomes unnecessary to explicitly initialize free_objectid
for a newly create fs root.
Reviewed-by: NJosef Bacik <josef@toxicpanda.com>
Signed-off-by: NNikolay Borisov <nborisov@suse.com>
Reviewed-by: NDavid Sterba <dsterba@suse.com>
Signed-off-by: NDavid Sterba <dsterba@suse.com>

fs/btrfs/disk-io.c

@@ -4767,10 +4767,10 @@ int btrfs_init_root_free_objectid(struct btrfs_root *root)
 		slot = path->slots[0] - 1;
 		l = path->nodes[0];
 		btrfs_item_key_to_cpu(l, &found_key, slot);
-		root->free_objectid = max_t(u64, found_key.objectid,
-				  BTRFS_FIRST_FREE_OBJECTID - 1);
+		root->free_objectid = max_t(u64, found_key.objectid + 1,
+					    BTRFS_FIRST_FREE_OBJECTID);
 	} else {
-		root->free_objectid = BTRFS_FIRST_FREE_OBJECTID - 1;
+		root->free_objectid = BTRFS_FIRST_FREE_OBJECTID;
 	}
 	ret = 0;
 error:
@@ -4791,7 +4791,7 @@ int btrfs_get_free_objectid(struct btrfs_root *root, u64 *objectid)
 		goto out;
 	}
 
-	*objectid = ++root->free_objectid;
+	*objectid = root->free_objectid++;
 	ret = 0;
 out:
 	mutex_unlock(&root->objectid_mutex);

fs/btrfs/inode.c

@@ -8598,9 +8598,13 @@ int btrfs_create_subvol_root(struct btrfs_trans_handle *trans,
 	struct inode *inode;
 	int err;
 	u64 index = 0;
+	u64 ino;
+
+	err = btrfs_get_free_objectid(new_root, &ino);
+	if (err < 0)
+		return err;
 
-	inode = btrfs_new_inode(trans, new_root, NULL, "..", 2,
-				new_dirid, new_dirid,
+	inode = btrfs_new_inode(trans, new_root, NULL, "..", 2, ino, ino,
 				S_IFDIR | (~current_umask() & S_IRWXUGO),
 				&index);
 	if (IS_ERR(inode))

fs/btrfs/ioctl.c

@@ -724,10 +724,6 @@ static noinline int create_subvol(struct inode *dir,
 		goto fail;
 	}
 
-	mutex_lock(&new_root->objectid_mutex);
-	new_root->free_objectid = new_dirid;
-	mutex_unlock(&new_root->objectid_mutex);
-
 	/*
 	 * insert the directory item
 	 */