Re: [PATCH v5 4/4] rust: samples: Add debugfs sample

[Danilo Krummrich <dakr@kernel.org>]

On Thu, May 15, 2025 at 01:43:09PM +0200, Greg Kroah-Hartman wrote:
> On Thu, May 15, 2025 at 10:59:44AM +0200, Benno Lossin wrote:
> > On Wed May 14, 2025 at 11:55 PM CEST, Matthew Maurer wrote:
> > > On Wed, May 14, 2025 at 2:07 AM Danilo Krummrich <dakr@kernel.org> wrote:
> > >> However, I really think we should keep the code as it is in this version and
> > >> just don't provide an example that utilizes ManuallyDrop and forget().
> > >>
> > >> I don't see how the idea of "manually dropping" (sub-)directories and files
> > >> provides any real value compared to just storing their instance in a driver
> > >> structure as long as they should stay alive, which is much more intuitive
> > >> anyways.
> > >
> > > We can't easily do this, because dropping a root directory recursively
> > > drops everything underneath it. This means that if I have
> > >
> > > foo/
> > >   - bar/
> > >   - baz/
> > >
> > > Then my directory handle for `bar` have to be guaranteed to outlive my
> > > directory handle for `foo` so that I know it's didn't get deleted
> > > under me. This is why they have a borrow onto their parent directory.
> > > This borrow means that you can't (without `unsafe`, or something like
> > > `yoke`) keep handles to `foo` and `bar` in the same struct.
> > 
> > Is there no refcount that we can use instead of borrowing? I guess not,
> > since one can call `debugfs_remove`. What about a refcount on the rust
> > side? or is debugfs not used for "debugging" and needs to have the
> > performance of no refcount?
> 
> debugfs should never have any performance issues (i.e. you don't use it
> for performant things.)
> 
> So refcount away!  That should never be an issue.

Reference counting (parent) directories should lead to a much cleaner solution.

I mentioned that previously, but also said in that context that it's a bit
contrary to how the C API is utilized currently, which usually isn't desired.

However, if we're fine with that I think it's superior to the borrowing
solution, which requires keep(). IMHO keep() is a footgun in general, even if
not callable for "root" directories.