BTRFS_ROOT_QGROUP_FLUSHING
if (test_and_set_bit(BTRFS_ROOT_QGROUP_FLUSHING, &root->state)) {
!test_bit(BTRFS_ROOT_QGROUP_FLUSHING, &root->state));
clear_bit(BTRFS_ROOT_QGROUP_FLUSHING, &root->state);