btrfs: qgroup: allow to remove qgroup which has parent but no child.

When a qgroup has parents but no child, it should be removable in
Theory I think. But currently, we can not remove it when it has
either parent or child.

Example:
	# btrfs quota enable /mnt
	# btrfs qgroup create 1/0 /mnt
	# btrfs qgroup create 2/0 /mnt
	# btrfs qgroup assign 1/0 2/0 /mnt
	# btrfs qgroup show -pcre /mnt
qgroupid rfer  excl  max_rfer max_excl parent  child
-------- ----  ----  -------- -------- ------  -----
0/5      16384 16384 0        0        ---     ---
1/0      0     0     0        0        2/0     ---
2/0      0     0     0        0        ---     1/0

At this time, there is no subvol or qgroup depending on it.
Just a qgroup 2/0 is its parent, but 2/0 can work well without
1/0. So I think 1/0 should be removalbe. But:
	# btrfs qgroup destroy 1/0 /mnt
ERROR: unable to destroy quota group: Device or resource busy

This patch remove the check of qgroup->parent in removing it,
then we can remove a qgroup when it has a parent.

Signed-off-by: Dongsheng Yang <yangds.fnst@cn.fujitsu.com>
Signed-off-by: Chris Mason <clm@fb.com>

1 files changed, 25 insertions, 5 deletions

diff --git a/fs/btrfs/qgroup.c b/fs/btrfs/qgroup.c
index 4ec86b3eee21..464d1e237512 100644
--- a/fs/btrfs/qgroup.c
+++ b/fs/btrfs/qgroup.c
@@ -1048,7 +1048,7 @@ out:
         return ret;
 }
 
-int btrfs_del_qgroup_relation(struct btrfs_trans_handle *trans,
+int __del_qgroup_relation(struct btrfs_trans_handle *trans,
                               struct btrfs_fs_info *fs_info, u64 src, u64 dst)
 {
         struct btrfs_root *quota_root;
@@ -1058,7 +1058,6 @@ int btrfs_del_qgroup_relation(struct btrfs_trans_handle *trans,
         int ret = 0;
         int err;
 
-        mutex_lock(&fs_info->qgroup_ioctl_lock);
         quota_root = fs_info->quota_root;
         if (!quota_root) {
                 ret = -EINVAL;
@@ -1089,7 +1088,18 @@ exist:
         del_relation_rb(fs_info, src, dst);
         spin_unlock(&fs_info->qgroup_lock);
 out:
+        return ret;
+}
+
+int btrfs_del_qgroup_relation(struct btrfs_trans_handle *trans,
+                              struct btrfs_fs_info *fs_info, u64 src, u64 dst)
+{
+        int ret = 0;
+
+        mutex_lock(&fs_info->qgroup_ioctl_lock);
+        ret = __del_qgroup_relation(trans, fs_info, src, dst);
         mutex_unlock(&fs_info->qgroup_ioctl_lock);
+
         return ret;
 }
 
@@ -1132,6 +1142,7 @@ int btrfs_remove_qgroup(struct btrfs_trans_handle *trans,
 {
         struct btrfs_root *quota_root;
         struct btrfs_qgroup *qgroup;
+        struct btrfs_qgroup_list *list;
         int ret = 0;
 
         mutex_lock(&fs_info->qgroup_ioctl_lock);
@@ -1146,15 +1157,24 @@ int btrfs_remove_qgroup(struct btrfs_trans_handle *trans,
                 ret = -ENOENT;
                 goto out;
         } else {
-                /* check if there are no relations to this qgroup */
-                if (!list_empty(&qgroup->groups) ||
-                    !list_empty(&qgroup->members)) {
+                /* check if there are no children of this qgroup */
+                if (!list_empty(&qgroup->members)) {
                         ret = -EBUSY;
                         goto out;
                 }
         }
         ret = del_qgroup_item(trans, quota_root, qgroupid);
 
+        while (!list_empty(&qgroup->groups)) {
+                list = list_first_entry(&qgroup->groups,
+                                        struct btrfs_qgroup_list, next_group);
+                ret = __del_qgroup_relation(trans, fs_info,
+                                           qgroupid,
+                                           list->group->qgroupid);
+                if (ret)
+                        goto out;
+        }
+
         spin_lock(&fs_info->qgroup_lock);
         del_qgroup_rb(quota_root->fs_info, qgroupid);
         spin_unlock(&fs_info->qgroup_lock);