Overview of the main locks protecting various device-related structures.
Signed-off-by: David Sterba <[email protected]>
---
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 [email protected]
More majordomo info at http://vger.kernel.org/majordomo-info.html
