Overview of the main locks protecting various device-related structures. Signed-off-by: David Sterba <dsterba@xxxxxxxx> --- fs/btrfs/volumes.c | 66 ++++++++++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 66 insertions(+) diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c index 75aed8ec64bd..098affc58361 100644 --- a/fs/btrfs/volumes.c +++ b/fs/btrfs/volumes.c @@ -145,6 +145,72 @@ static int __btrfs_map_block(struct btrfs_fs_info *fs_info, struct btrfs_bio **bbio_ret, int mirror_num, int need_raid_map); +/* + * Device locking + * ============== + * + * There are several mutexes that protect manipulation of devices and low-level + * structures like chunks but not block groups, extents or files + * + * - uuid_mutex (global lock) + * + * protects the fs_uuids list that tracks all per-fs fs_devices, resulting + * from the SCAN_DEV ioctl registration or from mount either implicitly + * (the first device) or requested by the device= mount option + * + * the mutex can be very coarse and can cover long-running operations + * + * protects: updates to fs_devices counters like missing devices, rw devices, + * seeding, structure cloning, openning/closing devices at mount/umount time + * + * global::fs_devs - add, remove, updates to the global list + * + * does not protect: manipulation of the fs_devices::devices list! + * + * btrfs_device::name - renames (write side), read is RCU + * + * - fs_devices::device_list_mutex (per-fs, with RCU) + * + * protects updates to fs_devices::devices, ie. adding and deleting + * + * simple list traversal with read-only actions can be done with RCU + * protection + * + * may be used to exclude some operations from running concurrently without + * any modifications to the list (see write_all_supers) + * + * - volume_mutex + * + * coarse lock owned by a mounted filesystem; used to exclude some operations + * that cannot run in parallel and affect the higher-level properties of the + * filesystem like: device add/deleting/resize/replace, or balance + * + * - balance_mutex + * + * protects balance structures (status, state) and context accessed from + * several places (internally, ioctl) + * + * - chunk_mutex + * + * protects chunks, adding or removing during allocation, trim or when + * a new device is added/removed + * + * - cleaner_mutex + * + * a big lock that is held by the cleaner thread and prevents running + * subvolume cleaning together with relocation or delayed iputs + * + * + * Lock nesting + * ------------ + * + * uuid_mutex + * volume_mutex + * device_list_mutex + * chunk_mutex + * balance_mutex + * + */ DEFINE_MUTEX(uuid_mutex); static LIST_HEAD(fs_uuids); struct list_head *btrfs_get_fs_uuids(void) -- 2.14.0 -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
