Btrfs: eliminate the exceptional root_tree refs=0

The fact that btrfs_root_refs() returned 0 for the tree_root caused
bugs in the past, therefore it is set to 1 with this patch and
(hopefully) all affected code is adapted to this change.

I verified this change by temporarily adding WARN_ON() checks
everywhere where btrfs_root_refs() is used, checking whether the
logic of the code is changed by btrfs_root_refs() returning 1
instead of 0 for root->root_key.objectid == BTRFS_ROOT_TREE_OBJECTID.
With these added checks, I ran the xfstests './check -g auto'.

The two roots chunk_root and log_root_tree that are only referenced
by the superblock and the log_roots below the log_root_tree still
have btrfs_root_refs() == 0, only the tree_root is changed.

Signed-off-by: Stefan Behrens <sbehrens@giantdisaster.de>
Signed-off-by: Josef Bacik <jbacik@fusionio.com>
Signed-off-by: Chris Mason <chris.mason@fusionio.com>

3 files changed, 10 insertions, 15 deletions

diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
index 62176ad89846..f724397b396b 100644
--- a/fs/btrfs/disk-io.c
+++ b/fs/btrfs/disk-io.c
@@ -2670,6 +2670,7 @@ retry_root_backup:
 
         btrfs_set_root_node(&tree_root->root_item, tree_root->node);
         tree_root->commit_root = btrfs_root_node(tree_root);
+        btrfs_set_root_refs(&tree_root->root_item, 1);
 
         location.objectid = BTRFS_EXTENT_TREE_OBJECTID;
         location.type = BTRFS_ROOT_ITEM_KEY;
diff --git a/fs/btrfs/inode-map.c b/fs/btrfs/inode-map.c
index 2c66ddbbe670..d11e1c6610b6 100644
--- a/fs/btrfs/inode-map.c
+++ b/fs/btrfs/inode-map.c
@@ -412,8 +412,7 @@ int btrfs_save_ino_cache(struct btrfs_root *root,
                 return 0;
 
         /* Don't save inode cache if we are deleting this root */
-        if (btrfs_root_refs(&root->root_item) == 0 &&
-            root != root->fs_info->tree_root)
+        if (btrfs_root_refs(&root->root_item) == 0)
                 return 0;
 
         if (!btrfs_test_opt(root, INODE_MAP_CACHE))
diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
index 51e3afa78354..dfc60aa40332 100644
--- a/fs/btrfs/inode.c
+++ b/fs/btrfs/inode.c
@@ -4472,8 +4472,10 @@ void btrfs_evict_inode(struct inode *inode)
         trace_btrfs_inode_evict(inode);
 
         truncate_inode_pages(&inode->i_data, 0);
-        if (inode->i_nlink && (btrfs_root_refs(&root->root_item) != 0 ||
-                               btrfs_is_free_space_inode(inode)))
+        if (inode->i_nlink &&
+            ((btrfs_root_refs(&root->root_item) != 0 &&
+              root->root_key.objectid != BTRFS_ROOT_TREE_OBJECTID) ||
+             btrfs_is_free_space_inode(inode)))
                 goto no_delete;
 
         if (is_bad_inode(inode)) {
@@ -4490,7 +4492,8 @@ void btrfs_evict_inode(struct inode *inode)
         }
 
         if (inode->i_nlink > 0) {
-                BUG_ON(btrfs_root_refs(&root->root_item) != 0);
+                BUG_ON(btrfs_root_refs(&root->root_item) != 0 &&
+                       root->root_key.objectid != BTRFS_ROOT_TREE_OBJECTID);
                 goto no_delete;
         }
 
@@ -4731,14 +4734,7 @@ static void inode_tree_del(struct inode *inode)
         }
         spin_unlock(&root->inode_lock);
 
-        /*
-         * Free space cache has inodes in the tree root, but the tree root has a
-         * root_refs of 0, so this could end up dropping the tree root as a
-         * snapshot, so we need the extra !root->fs_info->tree_root check to
-         * make sure we don't drop it.
-         */
-        if (empty && btrfs_root_refs(&root->root_item) == 0 &&
-            root != root->fs_info->tree_root) {
+        if (empty && btrfs_root_refs(&root->root_item) == 0) {
                 synchronize_srcu(&root->fs_info->subvol_srcu);
                 spin_lock(&root->inode_lock);
                 empty = RB_EMPTY_ROOT(&root->inode_tree);
@@ -7857,8 +7853,7 @@ int btrfs_drop_inode(struct inode *inode)
                 return 1;
 
         /* the snap/subvol tree is on deleting */
-        if (btrfs_root_refs(&root->root_item) == 0 &&
-            root != root->fs_info->tree_root)
+        if (btrfs_root_refs(&root->root_item) == 0)
                 return 1;
         else
                 return generic_drop_inode(inode);