BTRFS Volume operations, Device Lists and Locks all in one page:
Devices are managed in two contexts, the scan context and the mounted
context. In scan context the threads originate from the btrfs_control
ioctl and in the mounted context the threads originates from the mount
point ioctl.
Apart from these two context, there also can be two transient state
where device state are transitioning from the scan to the mount context
or from the mount to the scan context.
Device List and Locks:-
Count: btrfs_fs_devices::num_devices
List : btrfs_fs_devices::devices -> btrfs_devices::dev_list
Lock : btrfs_fs_devices::device_list_mutex
Count: btrfs_fs_devices::rw_devices
List : btrfs_fs_devices::alloc_list -> btrfs_devices::dev_alloc_list
Lock : btrfs_fs_info::chunk_mutex
Lock: set_bit btrfs_fs_info::flags::BTRFS_FS_EXCL_OP
FSID List and Lock:-
Count : None
HEAD : Global::fs_uuids -> btrfs_fs_devices::fs_list
Lock : Global::uuid_mutex
After the fs_devices is mounted, the btrfs_fs_devices::opened > 0.
In the scan context we have the following device operations..
Device SCAN:- which creates the btrfs_fs_devices and its corresponding
btrfs_device entries, also checks and frees the duplicate device entries.
Lock: uuid_mutex
SCAN
if (found_duplicate && btrfs_fs_devices::opened == 0)
Free_duplicate
Unlock: uuid_mutex
Device READY:- check if the volume is ready. Also does an implicit scan
and duplicate device free as in Device SCAN.
Lock: uuid_mutex
SCAN
if (found_duplicate && btrfs_fs_devices::opened == 0)
Free_duplicate
Check READY
Unlock: uuid_mutex
Device FORGET:- (planned) free a given or all unmounted devices and
empty fs_devices if any.
Lock: uuid_mutex
if (found_duplicate && btrfs_fs_devices::opened == 0)
Free duplicate
Unlock: uuid_mutex
Device mount operation -> A Transient state leading to the mounted context
Lock: uuid_mutex
Find, SCAN, btrfs_fs_devices::opened++
Unlock: uuid_mutex
Device umount operation -> A transient state leading to the unmounted
context or scan context
Lock: uuid_mutex
btrfs_fs_devices::opened--
Unlock: uuid_mutex
In the mounted context we have the following device operations..
Device Rename through SCAN:- This is a special case where the device
path gets renamed after its been mounted. (Ubuntu changes the boot path
during boot up so we need this feature). Currently, this is part of
Device SCAN as above. And we need the locks as below, because the
dynamic disappearing device might cleanup the btrfs_device::name
Lock: btrfs_fs_devices::device_list_mutex
Rename
Unlock: btrfs_fs_devices::device_list_mutex
Commit Transaction:- Write All supers.
Lock: btrfs_fs_devices::device_list_mutex
Write all super of btrfs_devices::dev_list
Unlock: btrfs_fs_devices::device_list_mutex
Device add:- Add a new device to the existing mounted volume.
set_bit: btrfs_fs_info::flags::BTRFS_FS_EXCL_OP
Lock: btrfs_fs_devices::device_list_mutex
Lock: btrfs_fs_info::chunk_mutex
List_add btrfs_devices::dev_list
List_add btrfs_devices::dev_alloc_list
Unlock: btrfs_fs_info::chunk_mutex
Unlock: btrfs_fs_devices::device_list_mutex
Device remove:- Remove a device from the mounted volume.
set_bit: btrfs_fs_info::flags::BTRFS_FS_EXCL_OP
Lock: btrfs_fs_devices::device_list_mutex
Lock: btrfs_fs_info::chunk_mutex
List_del btrfs_devices::dev_list
List_del btrfs_devices::dev_alloc_list
Unlock: btrfs_fs_info::chunk_mutex
Unlock: btrfs_fs_devices::device_list_mutex
Device Replace:- Replace a device.
set_bit: btrfs_fs_info::flags::BTRFS_FS_EXCL_OP
Lock: btrfs_fs_devices::device_list_mutex
Lock: btrfs_fs_info::chunk_mutex
List_update btrfs_devices::dev_list
List_update btrfs_devices::dev_alloc_list
Unlock: btrfs_fs_info::chunk_mutex
Unlock: btrfs_fs_devices::device_list_mutex
Sprouting:- Add a RW device to the mounted RO seed device, so to make
the mount point writable.
The following steps are used to hold the seed and sprout fs_devices.
(first two steps are not necessary for the sprouting, they are there to
ensure the seed device remains scanned, and it might change)
. Clone the (mounted) fs_devices, lets call it as old_devices
. Now add old_devices to fs_uuids (yeah, there is duplicate fsid in the
list but we change the other fsid before we release the uuid_mutex, so
its fine).
. Alloc a new fs_devices, lets call it as seed_devices
. Copy fs_devices into the seed_devices
. Move fs_deviecs devices list into seed_devices
. Bring seed_devices to under fs_devices (fs_devices->seed = seed_devices)
. Assign a new FSID to the fs_devices and add the new writable device to
the fs_devices.
In the unmounted context the fs_devices::seed is always NULL.
We alloc the fs_devices::seed only at the time of mount and or at
sprouting. And free at the time of umount or if the seed device is
replaced or deleted.
Locks: Sprouting:
Lock: uuid_mutex <-- because fsid rename and Device SCAN
Reuses Device Add code
Locks: Splitting: (Delete OR Replace a seed device)
uuid_mutex is not required as fs_devices::seed which is local to
fs_devices is being altered.
Reuses Device replace code
Device resize:- Resize the given volume or device.
Lock: btrfs_fs_info::chunk_mutex
Update
Unlock: btrfs_fs_info::chunk_mutex
(Planned) Dynamic Device missing/reappearing:- A missing device might
reappear after its volume been mounted, we have the same btrfs_control
ioctl which does the scan of the reappearing device but in the mounted
context. In the contrary a device of a volume in a mounted context can
go missing as well, and still the volume will continue in the mounted
context.
Missing:
Lock: btrfs_fs_devices::device_list_mutex
Lock: btrfs_fs_info::chunk_mutex
List_del: btrfs_devices::dev_alloc_list
Close_bdev
btrfs_device::bdev == NULL
btrfs_device::name = NULL
set_bit BTRFS_DEV_STATE_MISSING
set_bit BTRFS_VOL_STATE_DEGRADED
Unlock: btrfs_fs_info::chunk_mutex
Unlock: btrfs_fs_devices::device_list_mutex
Reappearing:
Lock: btrfs_fs_devices::device_list_mutex
Lock: btrfs_fs_info::chunk_mutex
Open_bdev
btrfs_device::name = PATH
clear_bit BTRFS_DEV_STATE_MISSING
clear_bit BTRFS_VOL_STATE_DEGRADED
List_add: btrfs_devices::dev_alloc_list
set_bit BTRFS_VOL_STATE_RESILVERING
kthread_run HEALTH_CHECK
Unlock: btrfs_fs_info::chunk_mutex
Unlock: btrfs_fs_devices::device_list_mutex
-----------------------------------------------------------------------
Thanks, Anand
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