btrfs: qgroup: fix deadlock between rescan worker and remove qgroup

The commit e804861b ("btrfs: fix deadlock between quota disable and
qgroup rescan worker") by Kawasaki resolves deadlock between quota
disable and qgroup rescan worker. But also there is a deadlock case like
it. It's about enabling or disabling quota and creating or removing
qgroup. It can be reproduced in simple script below.

for i in {1..100}
do
    btrfs quota enable /mnt &
    btrfs qgroup create 1/0 /mnt &
    btrfs qgroup destroy 1/0 /mnt &
    btrfs quota disable /mnt &
done

Here's why the deadlock happens:

1) The quota rescan task is running.

2) Task A calls btrfs_quota_disable(), locks the qgroup_ioctl_lock
   mutex, and then calls btrfs_qgroup_wait_for_completion(), to wait for
   the quota rescan task to complete.

3) Task B calls btrfs_remove_qgroup() and it blocks when trying to lock
   the qgroup_ioctl_lock mutex, because it's being held by task A. At that
   point task B is holding a transaction handle for the current transaction.

4) The quota rescan task calls btrfs_commit_transaction(). This results
   in it waiting for all other tasks to release their handles on the
   transaction, but task B is blocked on the qgroup_ioctl_lock mutex
   while holding a handle on the transaction, and that mutex is being held
   by task A, which is waiting for the quota rescan task to complete,
   resulting in a deadlock between these 3 tasks.

To resolve this issue, the thread disabling quota should unlock
qgroup_ioctl_lock before waiting rescan completion. Move
btrfs_qgroup_wait_for_completion() after unlock of qgroup_ioctl_lock.

Fixes: e804861b ("btrfs: fix deadlock between quota disable and qgroup rescan worker")
CC: stable@vger.kernel.org # 5.4+
Reviewed-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: Shin'ichiro Kawasaki <shinichiro.kawasaki@wdc.com>
Signed-off-by: Sidong Yang <realwakka@gmail.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/qgroup.c

@@ -1196,6 +1196,14 @@ int btrfs_quota_disable(struct btrfs_fs_info *fs_info)
 	if (!fs_info->quota_root)
 		goto out;
 
+	/*
+	 * Unlock the qgroup_ioctl_lock mutex before waiting for the rescan worker to
+	 * complete. Otherwise we can deadlock because btrfs_remove_qgroup() needs
+	 * to lock that mutex while holding a transaction handle and the rescan
+	 * worker needs to commit a transaction.
+	 */
+	mutex_unlock(&fs_info->qgroup_ioctl_lock);
+
 	/*
 	 * Request qgroup rescan worker to complete and wait for it. This wait
 	 * must be done before transaction start for quota disable since it may
@@ -1203,7 +1211,6 @@ int btrfs_quota_disable(struct btrfs_fs_info *fs_info)
 	 */
 	clear_bit(BTRFS_FS_QUOTA_ENABLED, &fs_info->flags);
 	btrfs_qgroup_wait_for_completion(fs_info, false);
-	mutex_unlock(&fs_info->qgroup_ioctl_lock);
 
 	/*
 	 * 1 For the root item