Re: [PATCH] ocfs2: fix check if list iterator did find an element

[Joseph Qi]

On 3/20/22 4:31 AM, Jakob Koschel wrote:
> Instead of setting 'res' to NULL, it should only be set if
> the suitable element was found.
> 
> In the original code 'res' would have been set to an incorrect pointer
> if the list is empty.
> 
The logic before iteration can make sure track_list won't be empty.
Please refer the discussion via:
https://lore.kernel.org/ocfs2-devel/bd0ec87e-b490-83dc-2363-5e5342c59fa4@xxxxxxxxxxxxxxxxx/T/#m96d4397930201d83d68677c33a9721ae8dbd8f15

Thanks,
Joseph

> In preparation to limit the scope of the list iterator to the list
> traversal loop, use a dedicated pointer pointing to the found element [1].
> 
> Link: https://lore.kernel.org/all/YhdfEIwI4EdtHdym@xxxxxxxxx/
> Signed-off-by: Jakob Koschel <jakobkoschel@xxxxxxxxx>
> ---
>  fs/ocfs2/dlm/dlmdebug.c | 12 ++++++------
>  1 file changed, 6 insertions(+), 6 deletions(-)
> 
> diff --git a/fs/ocfs2/dlm/dlmdebug.c b/fs/ocfs2/dlm/dlmdebug.c
> index d442cf5dda8a..be5e9ed7da8d 100644
> --- a/fs/ocfs2/dlm/dlmdebug.c
> +++ b/fs/ocfs2/dlm/dlmdebug.c
> @@ -541,7 +541,7 @@ static void *lockres_seq_start(struct seq_file *m, loff_t *pos)
>  	struct debug_lockres *dl = m->private;
>  	struct dlm_ctxt *dlm = dl->dl_ctxt;
>  	struct dlm_lock_resource *oldres = dl->dl_res;
> -	struct dlm_lock_resource *res = NULL;
> +	struct dlm_lock_resource *res = NULL, *iter;
>  	struct list_head *track_list;
> 
>  	spin_lock(&dlm->track_lock);
> @@ -556,11 +556,11 @@ static void *lockres_seq_start(struct seq_file *m, loff_t *pos)
>  		}
>  	}
> 
> -	list_for_each_entry(res, track_list, tracking) {
> -		if (&res->tracking == &dlm->tracking_list)
> -			res = NULL;
> -		else
> -			dlm_lockres_get(res);
> +	list_for_each_entry(iter, track_list, tracking) {
> +		if (&iter->tracking != &dlm->tracking_list) {
> +			dlm_lockres_get(iter);
> +			res = iter;
> +		}
>  		break;
>  	}
>  	spin_unlock(&dlm->track_lock);
> 
> base-commit: 34e047aa16c0123bbae8e2f6df33e5ecc1f56601
> --
> 2.25.1