Re: [PATCH] Btrfs: prevent send failures and crashes due to concurrent relocation

[David Sterba <dsterba@suse.cz>]

On Mon, Apr 22, 2019 at 04:44:09PM +0100, fdmanana@kernel.org wrote:
> From: Filipe Manana <fdmanana@suse.com>
> --- a/fs/btrfs/send.c
> +++ b/fs/btrfs/send.c
> @@ -6869,9 +6869,23 @@ long btrfs_ioctl_send(struct file *mnt_file, struct btrfs_ioctl_send_args *arg)
>  	if (ret)
>  		goto out;
>  
> +	mutex_lock(&fs_info->balance_mutex);
> +	if (test_bit(BTRFS_FS_BALANCE_RUNNING, &fs_info->flags)) {
> +		mutex_unlock(&fs_info->balance_mutex);
> +		btrfs_warn_rl(fs_info,
> +	      "Can not run send because a balance operation is in progress");
> +		ret = -EAGAIN;
> +		goto out;
> +	}
> +	fs_info->send_in_progress++;
> +	mutex_unlock(&fs_info->balance_mutex);

This would be better in a helper that hides that the balance mutex from
send.

eg.

	if (!btrfs_send_can_start(fs_info)
		return -EAGAIN;

> +
>  	current->journal_info = BTRFS_SEND_TRANS_STUB;
>  	ret = send_subvol(sctx);
>  	current->journal_info = NULL;
> +	mutex_lock(&fs_info->balance_mutex);
> +	fs_info->send_in_progress--;
> +	mutex_unlock(&fs_info->balance_mutex);

	btrfs_send_end();

>  	if (ret < 0)
>  		goto out;
>  
> diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
> index db934ceae9c1..8145b62e3912 100644
> --- a/fs/btrfs/volumes.c
> +++ b/fs/btrfs/volumes.c
> @@ -4203,6 +4203,14 @@ int btrfs_balance(struct btrfs_fs_info *fs_info,
>  			   get_raid_name(meta_index), get_raid_name(data_index));
>  	}
>  
> +	if (fs_info->send_in_progress) {
> +		btrfs_warn_rl(fs_info,
> +"Can not run balance while send operations are in progress (%d in progress)",
> +			      fs_info->send_in_progress);
> +		ret = -EAGAIN;
> +		goto out;
> +	}

Similar here.

As the operation compatibility is done on the filesystem level, it would
be better to hide all the logic in helpers, now that there's more than
the per-subvolume send_in_progress.