BTRFS_DEV_STATE_FLUSH_SENT
set_bit(BTRFS_DEV_STATE_FLUSH_SENT, &device->dev_state);
if (!test_and_clear_bit(BTRFS_DEV_STATE_FLUSH_SENT, &device->dev_state))
WARN_ON(test_bit(BTRFS_DEV_STATE_FLUSH_SENT, &device->dev_state));