Re: [PATCH 2/3] btrfs: Don't use objectid_mutex during mount

[David Sterba <dsterba@suse.cz>]

On Thu, Oct 10, 2019 at 06:06:46PM +0300, Nikolay Borisov wrote:
> Since the filesystem is not well formed and no trees are loaded it's
> pointless holding the objectid_mutex. Just remove its usage.

Yes.

There's a case when the lock should be kept even in that phase, so we
could add a lockdep assertion, but I found btrfs_recover_log_trees with
a comment why the lock is not needed. Given that
btrfs_find_highest_objectid is used only in a hadnful places, I think
we're fine with just the comments.

> Signed-off-by: Nikolay Borisov <nborisov@suse.com>
> ---
>  fs/btrfs/disk-io.c | 3 ---
>  1 file changed, 3 deletions(-)
> 
> diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
> index b850988023aa..72580eb6b706 100644
> --- a/fs/btrfs/disk-io.c
> +++ b/fs/btrfs/disk-io.c
> @@ -2603,17 +2603,14 @@ int __cold init_tree_roots(struct btrfs_fs_info *fs_info)
>  		tree_root->commit_root = btrfs_root_node(tree_root);
>  		btrfs_set_root_refs(&tree_root->root_item, 1);
>  
> -		mutex_lock(&tree_root->objectid_mutex);

		/*
		 * We don't need to hold objectid_mutex because ...
		 */

>  		ret = btrfs_find_highest_objectid(tree_root,
>  						&tree_root->highest_objectid);
>  		if (ret) {
> -			mutex_unlock(&tree_root->objectid_mutex);
>  			handle_error = true;
>  			continue;
>  		}