Re: [PATCH] fs: dlm: Remove impossible to hit if statement

[Al Viro <viro@zeniv.linux.org.uk>]

On Sun, Nov 05, 2023 at 11:11:25PM +0000, Al Viro wrote:
> Sure, if you hit even a single iteration of that loop, you will
> have memb guaranteed to be non-NULL.  Therefore, to complete the
> proof you only need to consider what happens if there is not
> a single iteration.  Which is to say, what happens if the list
> is empty.  Well, either memb is uninitialized, or there is an
> intialization somewhere upstream.  Declaration is not far before
> that loop, and it is
>         struct dlm_member *memb = NULL;
> Er...  So for that change to be correct you need to show that
> the list (ls->ls_nodes) can not be empty here.  Unfortunately,
> it looks like it very much can be empty, seeing that this
> is apparently the only place where elements are added to
> the list in question.  So on the very first call it will
> hit your "impossible to hit" case.  Which leads to...
> 
> * the patch had apparently never been tested.

Looking at the uses of ->ls_nodes,  I wonder if xarray would be
a better fit here.  Might be interesting to investigate...