Re: [PATCH 8/8] btrfs: set the lockdep class for log tree extent buffers

[Nikolay Borisov <nborisov@suse.com>]

On 20.08.20 г. 18:18 ч., Josef Bacik wrote:
> These are special extent buffers that get rewound in order to lookup
> the state of the tree at a specific point in time.  As such they do not
> go through the normal initialization paths that set their lockdep class,
> so handle them appropriately when they are created and before they are
> locked.
> 
> Signed-off-by: Josef Bacik <josef@toxicpanda.com>

Setting the lockdep class is fine, however in get_old_root your also
reduce the scope of the eb's read lock. That's fine since in the if
(old_root) branch we only set some values to the eb, which is guaranteed
to be a clone/private copy i.e no other references.

Reviewed-by: Nikolay Borisov <nborisov@suse.com>

> ---
>  fs/btrfs/ctree.c | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/btrfs/ctree.c b/fs/btrfs/ctree.c
> index 09a0dc690a17..e0fd8a34efcc 100644
> --- a/fs/btrfs/ctree.c
> +++ b/fs/btrfs/ctree.c
> @@ -1297,6 +1297,8 @@ tree_mod_log_rewind(struct btrfs_fs_info *fs_info, struct btrfs_path *path,
>  	btrfs_tree_read_unlock_blocking(eb);
>  	free_extent_buffer(eb);
>  
> +	btrfs_set_buffer_lockdep_class(btrfs_header_owner(eb_rewin),
> +				       eb_rewin, btrfs_header_level(eb_rewin));
>  	btrfs_tree_read_lock(eb_rewin);
>  	__tree_mod_log_rewind(fs_info, eb_rewin, time_seq, tm);
>  	WARN_ON(btrfs_header_nritems(eb_rewin) >
> @@ -1370,7 +1372,6 @@ get_old_root(struct btrfs_root *root, u64 time_seq)
>  
>  	if (!eb)
>  		return NULL;
> -	btrfs_tree_read_lock(eb);
>  	if (old_root) {
>  		btrfs_set_header_bytenr(eb, eb->start);
>  		btrfs_set_header_backref_rev(eb, BTRFS_MIXED_BACKREF_REV);
> @@ -1378,6 +1379,9 @@ get_old_root(struct btrfs_root *root, u64 time_seq)
>  		btrfs_set_header_level(eb, old_root->level);
>  		btrfs_set_header_generation(eb, old_generation);
>  	}
> +	btrfs_set_buffer_lockdep_class(btrfs_header_owner(eb), eb,
> +				       btrfs_header_level(eb));
> +	btrfs_tree_read_lock(eb);
>  	if (tm)
>  		__tree_mod_log_rewind(fs_info, eb, time_seq, tm);
>  	else
>