Btrfs: don't write any data into a readonly device when scrub

We should not write data into a readonly device especially seed device when
doing scrub, skip those devices.
Signed-off-by: Miao Xie <miaox@cn.fujitsu.com>
Reviewed-by: David Sterba <dsterba@suse.cz>
Signed-off-by: Chris Mason <clm@fb.com>

fs/btrfs/scrub.c

@@ -2904,6 +2904,7 @@ int btrfs_scrub_dev(struct btrfs_fs_info *fs_info, u64 devid, u64 start,
 	struct scrub_ctx *sctx;
 	int ret;
 	struct btrfs_device *dev;
+	struct rcu_string *name;
 
 	if (btrfs_fs_closing(fs_info))
 		return -EINVAL;
@@ -2965,6 +2966,16 @@ int btrfs_scrub_dev(struct btrfs_fs_info *fs_info, u64 devid, u64 start,
 		return -ENODEV;
 	}
 
+	if (!is_dev_replace && !readonly && !dev->writeable) {
+		mutex_unlock(&fs_info->fs_devices->device_list_mutex);
+		rcu_read_lock();
+		name = rcu_dereference(dev->name);
+		btrfs_err(fs_info, "scrub: device %s is not writable",
+			  name->str);
+		rcu_read_unlock();
+		return -EROFS;
+	}
+
 	mutex_lock(&fs_info->scrub_lock);
 	if (!dev->in_fs_metadata || dev->is_tgtdev_for_dev_replace) {
 		mutex_unlock(&fs_info->scrub_lock);