btrfs: remove wrong use of volume_mutex from btrfs_dev_replace_start

The volume mutex does not protect against anything in this case, the
comment about scrub is right but not related to locking and looks
confusing. The comment in btrfs_find_device_missing_or_by_path is wrong
and confusing too.

The device_list_mutex is not held here to protect device lookup, but in
this case device replace cannot run in parallel with device removal (due
to exclusive op protection), so we don't need further locking here.
Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/dev-replace.c

@@ -414,18 +414,13 @@ int btrfs_dev_replace_start(struct btrfs_fs_info *fs_info,
 	struct btrfs_device *tgt_device = NULL;
 	struct btrfs_device *src_device = NULL;
 
-	/* the disk copy procedure reuses the scrub code */
-	mutex_lock(&fs_info->volume_mutex);
 	ret = btrfs_find_device_by_devspec(fs_info, srcdevid,
 					    srcdev_name, &src_device);
-	if (ret) {
-		mutex_unlock(&fs_info->volume_mutex);
+	if (ret)
 		return ret;
-	}
 
 	ret = btrfs_init_dev_replace_tgtdev(fs_info, tgtdev_name,
 					    src_device, &tgt_device);
-	mutex_unlock(&fs_info->volume_mutex);
 	if (ret)
 		return ret;
 

fs/btrfs/volumes.c

@@ -2218,10 +2218,6 @@ int btrfs_find_device_missing_or_by_path(struct btrfs_fs_info *fs_info,
 		struct btrfs_device *tmp;
 
 		devices = &fs_info->fs_devices->devices;
-		/*
-		 * It is safe to read the devices since the volume_mutex
-		 * is held by the caller.
-		 */
 		list_for_each_entry(tmp, devices, dev_list) {
 			if (test_bit(BTRFS_DEV_STATE_IN_FS_METADATA,
 					&tmp->dev_state) && !tmp->bdev) {