Re: [PATCH v16 2/4] rust: types: add `ForeignOwnable::PointedTo`

[Danilo Krummrich <dakr@kernel.org>]

On Mon, Feb 17, 2025 at 09:21:00AM -0500, Tamir Duberstein wrote:
> On Mon, Feb 17, 2025 at 9:15 AM Danilo Krummrich <dakr@kernel.org> wrote:
> >
> > On Mon, Feb 17, 2025 at 09:02:12AM -0500, Tamir Duberstein wrote:
> > > > > diff --git a/rust/kernel/pci.rs b/rust/kernel/pci.rs
> > > > > index 6c3bc14b42ad..eb25fabbff9c 100644
> > > > > --- a/rust/kernel/pci.rs
> > > > > +++ b/rust/kernel/pci.rs
> > > > > @@ -73,6 +73,7 @@ extern "C" fn probe_callback(
> > > > >          match T::probe(&mut pdev, info) {
> > > > >              Ok(data) => {
> > > > >                  let data = data.into_foreign();
> > > > > +                let data = data.cast();
> > > >
> > > > Same here and below, see also [2].
> > >
> > > You're the maintainer,
> >
> > This isn't true. I'm the original author, but I'm not an official maintainer of
> > this code. :)
> >
> > > so I'll do what you ask here as well. I did it
> > > this way because it avoids shadowing the git history with this change,
> > > which I thought was the dominant preference.
> >
> > As mentioned in [2], if you do it the other way around first the "rust: types:
> > add `ForeignOwnable::PointedTo`" patch and then the conversion to cast() it's
> > even cleaner and less code to change.
> 
> This is true for the two instances of `as _`,

Yes, those are the ones I talk about.

> but not for all the
> other instances where currently there's no cast, but one is now
> needed.
> 
> > >
> > > > I understand you like this style and I'm not saying it's wrong or forbidden and
> > > > for code that you maintain such nits are entirely up to you as far as I'm
> > > > concerned.
> > > >
> > > > But I also don't think there is a necessity to convert things to your preference
> > > > wherever you touch existing code.
> > >
> > > This isn't a conversion, it's a choice made specifically to avoid
> > > touching code that doesn't need to be touched (in this instance).
> >
> > See above.
> 
> This doesn't address my point. I claim that
> 
> @@ -246,6 +248,7 @@ impl<T: MiscDevice> VtableHelper<T> {
>  ) -> c_int {
>      // SAFETY: The release call of a file owns the private data.
>      let private = unsafe { (*file).private_data };
> +    let private = private.cast();
>      // SAFETY: The release call of a file owns the private data.
>      let ptr = unsafe { <T::Ptr as ForeignOwnable>::from_foreign(private) };
> 
> is a better diff than
> 
> @@ -245,7 +245,7 @@ impl<T: MiscDevice> VtableHelper<T> {
>      file: *mut bindings::file,
>  ) -> c_int {
>      // SAFETY: The release call of a file owns the private data.
> -    let private = unsafe { (*file).private_data };
> +    let private = unsafe { (*file).private_data }.cast();
>      // SAFETY: The release call of a file owns the private data.
>      let ptr = unsafe { <T::Ptr as ForeignOwnable>::from_foreign(private) };
> 
> because it doesn't acquire the git blame on the existing line.

I disagree with the *rationale*, because it would also mean that if I have

  let result = a + b;

and it turns out that we're off by one later on, it'd be reasonable to change it
to

  let result = a - b;
  let result = result + 1;

in order to not acquire the git blame of the existing line.

> 
> > >
> > > > I already explicitly asked you not to do so in [3] and yet you did so while
> > > > keeping my ACK. :(
> > > >
> > > > (Only saying the latter for reference, no need to send a new version of [3],
> > > > otherwise I would have replied.)
> > > >
> > > > [2] https://lore.kernel.org/rust-for-linux/Z7MYNQgo28sr_4RS@cassiopeiae/
> > > > [3] https://lore.kernel.org/rust-for-linux/20250213-aligned-alloc-v7-1-d2a2d0be164b@gmail.com/
> > >
> > > I will drop [2] and leave the `as _` casts in place to minimize
> > > controversy here.
> >
> > As mentioned I think the conversion to cast() is great, just do it after this
> > one and keep it a single line -- no controversy. :)
> 
> The code compiles either way, so I'll leave it untouched rather than
> risk being scolded for sneaking unrelated changes.

Again, I never did that, but as already mentioned if it came across this way,
please consider that I tell you now, that it wasn't meant to be.

You're free to do the change (I encourage that), but that's of course up to you.

Subsequently, I kindly ask you though to abstain from saying that I accused you
of something or do scold you. Thanks!