Re: [PATCH] btrfs: fix deadlock when writing out space cache

[David Sterba <dsterba@suse.cz>]

On Wed, Nov 15, 2017 at 04:20:52PM -0500, Josef Bacik wrote:
> From: Josef Bacik <jbacik@fb.com>
> 
> If we fail to prepare our pages for whatever reason (out of memory in
> our case) we need to make sure to drop the block_group->data_rwsem,
> otherwise hilarity ensues.
> 
> Signed-off-by: Josef Bacik <jbacik@fb.com>
> ---
>  fs/btrfs/free-space-cache.c | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/btrfs/free-space-cache.c b/fs/btrfs/free-space-cache.c
> index cdc9f4015ec3..a6c643275210 100644
> --- a/fs/btrfs/free-space-cache.c
> +++ b/fs/btrfs/free-space-cache.c
> @@ -1263,8 +1263,12 @@ static int __btrfs_write_out_cache(struct btrfs_root *root, struct inode *inode,
>  
>  	/* Lock all pages first so we can lock the extent safely. */
>  	ret = io_ctl_prepare_pages(io_ctl, inode, 0);
> -	if (ret)
> +	if (ret) {
> +		if (block_group &&
> +		    (block_group->flags & BTRFS_BLOCK_GROUP_DATA))
> +			up_write(&block_group->data_rwsem);
>  		goto out;

The unlocking sequence is in the exit block but does not have a label.
It would be better to reuse the code.