From 9c6b1c4de1c64cd35d029ed6e0428d0a57fa632b Mon Sep 17 00:00:00 2001 From: David Sterba Date: Fri, 16 Jun 2017 22:30:00 +0200 Subject: [PATCH] btrfs: document device locking Overview of the main locks protecting various device-related structures. Signed-off-by: David Sterba --- fs/btrfs/volumes.c | 65 ++++++++++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 65 insertions(+) diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c index 3fa83ff..372f515 100644 --- a/fs/btrfs/volumes.c +++ b/fs/btrfs/volumes.c @@ -145,6 +145,71 @@ 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) -- 2.7.4