Btrfs: avoid races between super writeout and device list updates
On multi-device filesystems, btrfs writes supers to all of the devices
before considering a sync complete. There wasn't any additional
locking between super writeout and the device list management code
because device management was done inside a transaction and
super writeout only happened with no transation writers running.
With the btrfs fsync log and other async transaction updates, this
has been racey for some time. This adds a mutex to protect
the device list. The existing volume mutex could not be reused due to
transaction lock ordering requirements.
Signed-off-by: Chris Mason <chris.mason@oracle.com>
diff --git a/fs/btrfs/volumes.h b/fs/btrfs/volumes.h
index 3c1f731..5139a83 100644
--- a/fs/btrfs/volumes.h
+++ b/fs/btrfs/volumes.h
@@ -96,7 +96,12 @@
u64 rw_devices;
u64 total_rw_bytes;
struct block_device *latest_bdev;
- /* all of the devices in the FS */
+
+ /* all of the devices in the FS, protected by a mutex
+ * so we can safely walk it to write out the supers without
+ * worrying about add/remove by the multi-device code
+ */
+ struct mutex device_list_mutex;
struct list_head devices;
/* devices not currently being allocated */