btrfs: add comment about BTRFS_FS_EXCL_OP

 *     device_list_mutex

 *       chunk_mutex

 *     balance_mutex

 *

 *

 * Exclusive operations, BTRFS_FS_EXCL_OP

 * ======================================

 *

 * Maintains the exclusivity of the following operations that apply to the

 * whole filesystem and cannot run in parallel.

 *

 * - Balance (*)

 * - Device add

 * - Device remove

 * - Device replace (*)

 * - Resize

 *

 * The device operations (as above) can be in one of the following states:

 *

 * - Running state

 * - Paused state

 * - Completed state

 *

 * Only device operations marked with (*) can go into the Paused state for the

 * following reasons:

 *

 * - ioctl (only Balance can be Paused through ioctl)

 * - filesystem remounted as read-only

 * - filesystem unmounted and mounted as read-only

 * - system power-cycle and filesystem mounted as read-only

 * - filesystem or device errors leading to forced read-only

 *

 * BTRFS_FS_EXCL_OP flag is set and cleared using atomic operations.

 * During the course of Paused state, the BTRFS_FS_EXCL_OP remains set.

 * A device operation in Paused or Running state can be canceled or resumed

 * either by ioctl (Balance only) or when remounted as read-write.

 * BTRFS_FS_EXCL_OP flag is cleared when the device operation is canceled or

 * completed.

 */

DEFINE_MUTEX(uuid_mutex);