btrfs: fix device_list_add() missing device_list_mutex()

When the device is added or if the device pointer is retrieved
for writing, make sure that device_list_mutex is held. Also
make sure it is held when we check fs_devices::opened and
fs_devices::total_devices is updated.

Signed-off-by: Anand Jain <[email protected]>

fs/btrfs/volumes.c

@@ -762,36 +762,40 @@ static noinline struct btrfs_device *device_list_add(const char *path,
 		if (IS_ERR(fs_devices))
 			return ERR_CAST(fs_devices);
 
+		mutex_lock(&fs_devices->device_list_mutex);
 		list_add(&fs_devices->fs_list, &fs_uuids);
 
 		device = NULL;
 	} else {
+		mutex_lock(&fs_devices->device_list_mutex);
 		device = find_device(fs_devices, devid,
 				disk_super->dev_item.uuid);
 	}
 
 	if (!device) {
-		if (fs_devices->opened)
+		if (fs_devices->opened) {
+			mutex_unlock(&fs_devices->device_list_mutex);
 			return ERR_PTR(-EBUSY);
+		}
 
 		device = btrfs_alloc_device(NULL, &devid,
 					    disk_super->dev_item.uuid);
 		if (IS_ERR(device)) {
+			mutex_unlock(&fs_devices->device_list_mutex);
 			/* we can safely leave the fs_devices entry around */
 			return device;
 		}
 
 		name = rcu_string_strdup(path, GFP_NOFS);
 		if (!name) {
 			btrfs_free_device(device);
+			mutex_unlock(&fs_devices->device_list_mutex);
 			return ERR_PTR(-ENOMEM);
 		}
 		rcu_assign_pointer(device->name, name);
 
-		mutex_lock(&fs_devices->device_list_mutex);
 		list_add_rcu(&device->dev_list, &fs_devices->devices);
 		fs_devices->num_devices++;
-		mutex_unlock(&fs_devices->device_list_mutex);
 
 		device->fs_devices = fs_devices;
 		*new_device_added = true;
@@ -838,12 +842,15 @@ static noinline struct btrfs_device *device_list_add(const char *path,
 			 * with larger generation number or the last-in if
 			 * generation are equal.
 			 */
+			mutex_unlock(&fs_devices->device_list_mutex);
 			return ERR_PTR(-EEXIST);
 		}
 
 		name = rcu_string_strdup(path, GFP_NOFS);
-		if (!name)
+		if (!name) {
+			mutex_unlock(&fs_devices->device_list_mutex);
 			return ERR_PTR(-ENOMEM);
+		}
 		rcu_string_free(device->name);
 		rcu_assign_pointer(device->name, name);
 		if (test_bit(BTRFS_DEV_STATE_MISSING, &device->dev_state)) {
@@ -863,6 +870,7 @@ static noinline struct btrfs_device *device_list_add(const char *path,
 
 	fs_devices->total_devices = btrfs_super_num_devices(disk_super);
 
+	mutex_unlock(&fs_devices->device_list_mutex);
 	return device;
 }