[PATCH v2 1/3] btrfs: always remove quota_root from dirty_cowonly_roots

[Qu Wenruo <wqu@suse.com>]

[ERROR HANDLING QUIRK]
When btrfs_quota_disable() failed at btrfs_clean_quota_tree() or
btrfs_del_root(), the quota_root->dirty_list is not removed from
fs_info->dirty_cowonly_roots, but the quota_root itself will be freed.

This leaves a dangling pointer in the dirty_cowonly_roots.

This is not a huge problem, as the transaction is aborted thus no one
will read the dirty_cowonly_roots for now.

Furthermore, quota_root should never be logged nor have qgroup swapped
tree blocks, thus it won't cause leaks from extent io tree nor qgroup
swapped blocks.

[ENHANCEMENT]
For the incoming cleanup of dirty_cowonly_roots during
btrfs_free_fs_roots(), any root that is still linked to
dirty_cowonly_roots will be read again, and this existing dangling
pointer is going to cause problem.

To prepare for that cleanup, move the list_del() call into the out
label, so that even if the transaction is aborted, we still unlink
quota_root from dirty_cowonly_roots before freeing it.

Assisted-by: LLM (exposing the bug during review)
Signed-off-by: Qu Wenruo <wqu@suse.com>
---
 fs/btrfs/qgroup.c | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/fs/btrfs/qgroup.c b/fs/btrfs/qgroup.c
index a0651fbfe778..8bd3abdf4805 100644
--- a/fs/btrfs/qgroup.c
+++ b/fs/btrfs/qgroup.c
@@ -1440,10 +1440,6 @@ int btrfs_quota_disable(struct btrfs_fs_info *fs_info)
 		goto out;
 	}
 
-	spin_lock(&fs_info->trans_lock);
-	list_del(&quota_root->dirty_list);
-	spin_unlock(&fs_info->trans_lock);
-
 	btrfs_tree_lock(quota_root->node);
 	btrfs_clear_buffer_dirty(trans, quota_root->node);
 	btrfs_tree_unlock(quota_root->node);
@@ -1454,6 +1450,12 @@ int btrfs_quota_disable(struct btrfs_fs_info *fs_info)
 		btrfs_abort_transaction(trans, ret);
 
 out:
+	if (quota_root) {
+		spin_lock(&fs_info->trans_lock);
+		list_del(&quota_root->dirty_list);
+		spin_unlock(&fs_info->trans_lock);
+	}
+
 	btrfs_put_root(quota_root);
 	mutex_unlock(&fs_info->qgroup_ioctl_lock);
 	if (ret && trans)
-- 
2.56.0