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

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Message-ID: <2025051524-festival-afterglow-8483@gregkh>
Date: Thu, 15 May 2025 13:43:09 +0200
From: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To: Benno Lossin <lossin@...nel.org>
Cc: Matthew Maurer <mmaurer@...gle.com>, Danilo Krummrich <dakr@...nel.org>,
	Miguel Ojeda <ojeda@...nel.org>,
	Alex Gaynor <alex.gaynor@...il.com>,
	Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>,
	Björn Roy Baron <bjorn3_gh@...tonmail.com>,
	Benno Lossin <benno.lossin@...ton.me>,
	Andreas Hindborg <a.hindborg@...nel.org>,
	Alice Ryhl <aliceryhl@...gle.com>, Trevor Gross <tmgross@...ch.edu>,
	"Rafael J. Wysocki" <rafael@...nel.org>,
	Sami Tolvanen <samitolvanen@...gle.com>,
	Timur Tabi <ttabi@...dia.com>, linux-kernel@...r.kernel.org,
	rust-for-linux@...r.kernel.org
Subject: Re: [PATCH v5 4/4] rust: samples: Add debugfs sample

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@...nel.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.

thanks,

greg k-h