From: Sidong Yang <realwakka@xxxxxxxxx> commit d4aef1e122d8bbdc15ce3bd0bc813d6b44a7d63a upstream. The commit e804861bd4e6 ("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: e804861bd4e6 ("btrfs: fix deadlock between quota disable and qgroup rescan worker") CC: stable@xxxxxxxxxxxxxxx # 5.4+ Reviewed-by: Filipe Manana <fdmanana@xxxxxxxx> Reviewed-by: Shin'ichiro Kawasaki <shinichiro.kawasaki@xxxxxxx> Signed-off-by: Sidong Yang <realwakka@xxxxxxxxx> Reviewed-by: David Sterba <dsterba@xxxxxxxx> Signed-off-by: David Sterba <dsterba@xxxxxxxx> Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> --- fs/btrfs/qgroup.c | 9 ++++++++- 1 file changed, 8 insertions(+), 1 deletion(-) --- a/fs/btrfs/qgroup.c +++ b/fs/btrfs/qgroup.c @@ -1117,13 +1117,20 @@ int btrfs_quota_disable(struct btrfs_fs_ 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 * deadlock with transaction by the qgroup rescan worker. */ 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 Patches currently in stable-queue which might be from realwakka@xxxxxxxxx are queue-5.4/btrfs-qgroup-fix-deadlock-between-rescan-worker-and-remove-qgroup.patch