Re: [PATCH 51/51] fs/zonefs: Fix sparse warnings in tracing code

[Luc Van Oostenryck <luc.vanoostenryck@gmail.com>]

On Sun, Jun 26, 2022 at 09:33:57AM -0700, Linus Torvalds wrote:
> On Sun, Jun 26, 2022 at 2:58 AM Luc Van Oostenryck
> <luc.vanoostenryck@gmail.com> wrote:
> >
> > What about I would add to sparse something to strip away the bitwise/
> > recover the underlying type? Something like __unbitwiseof() or
> > __underlying_typeof() (some better name is needed)?
> 
> Please no, we don't want to make random macros have to have sparse
> logic in them when it's not actually sparse-related.
> 
> I think it would be better if sparse just recognized some of these
> kinds of situation. In particular:

Yes, sure, it's ideal.
 
>  (a) for the casting part, I actually suspect we should drop the
> warning about castign integers to restricted types.
> 
> Note that this is actually one of the main causes of "__force" use in
> the kernel, with code like
> 
>         VM_FAULT_OOM            = (__force vm_fault_t)0x000001,
>         VM_FAULT_SIGBUS         = (__force vm_fault_t)0x000002,
>         VM_FAULT_MAJOR          = (__force vm_fault_t)0x000004,
>         VM_FAULT_WRITE          = (__force vm_fault_t)0x000008,

This example is about an enumeration. It's, IMO, a very special case
in its own. Two years ago or so, I had proposed to have 'bitwise'
enums where the cast was not needed. In itself is was very easy to do
but there was a lot of subtle issues about type attributes. I think
I've since solved these issues but on the way I've lost my motivation
for these bitwise enums. I'll take a look at it again.
 
> and I think that we could/should just say that "explicit casts of
> constants are ok".

I'm not convinced, for example when thinking about __be{16,32}.
But on the principle, I fully agree: unneeded casts should be avoided.
 
> That would remove two of the four warnings right there, and probably
> make bitwise types more convenient in general.
> 
> We already treat "0" as special (because for bitwise things, zero is
> kind of the universal constant), and we should continue to warn about
> _implicit_ casts of restricted types, but I think the use of "__force"
> in the kernel does show that the explicit casts are probably a bad
> idea.

Yes.

>  (b) I think we could also recognize "comparison of constants" to be
> something that doesn't necessarily require a warning.
> 
> And here in particular the "compare with zero" and "compare with all
> bits set" - which is exactly that "-1" case.
>
> In fact, there's a very good argument that "-1" is as special as zero
> is ("all bits set" vs "all bits clear"), so for that (a) case, I think
> at a _minimum_ we shouldn't warn about that particular constant.
> 
> So I think we could silence this sparse warning entirely, without
> really introducing any new syntax, and actually improving on how
> bitwise works.

Yes, indeed.

-- Luc