(resent to list)

On Mon, July 01, 2013 at 22:12 (+0200), Josef Bacik wrote:
> There is another bug in the tree mod log stuff in that we're calling
> tree_mod_log_free_eb every single time a block is cow'ed.  The problem with 
> this
> is that if this block is shared by multiple snapshots we will call this 
> multiple
> times per block, so if we go to rewind the mod log for this block we'll 
> BUG_ON()
> in __tree_mod_log_rewind because we try to rewind a free twice.  We only want 
> to
> call tree_mod_log_free_eb if we are actually freeing the block.  With this 
> patch
> I no longer hit the panic in __tree_mod_log_rewind.  Thanks,
> 
> Signed-off-by: Josef Bacik <jba...@fusionio.com>

Strange that never really popped up largely so far, should be quite easy to hit.
Anyway,

Reviewed-by: Jan Schmidt <list.bt...@jan-o-sch.net>

> ---
>  fs/btrfs/ctree.c |    3 ++-
>  1 files changed, 2 insertions(+), 1 deletions(-)
> 
> diff --git a/fs/btrfs/ctree.c b/fs/btrfs/ctree.c
> index 32e30ad..127e1fd 100644
> --- a/fs/btrfs/ctree.c
> +++ b/fs/btrfs/ctree.c
> @@ -1093,7 +1093,8 @@ static noinline int __btrfs_cow_block(struct 
> btrfs_trans_handle *trans,
>               btrfs_set_node_ptr_generation(parent, parent_slot,
>                                             trans->transid);
>               btrfs_mark_buffer_dirty(parent);
> -             tree_mod_log_free_eb(root->fs_info, buf);
> +             if (last_ref)
> +                     tree_mod_log_free_eb(root->fs_info, buf);
>               btrfs_free_tree_block(trans, root, buf, parent_start,
>                                     last_ref);
>       }
> 
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