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

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Message-Id: <D9WM0BP5446N.1NVNDCZ4Y2QN1@kernel.org>
Date: Thu, 15 May 2025 10:59:44 +0200
From: "Benno Lossin" <lossin@...nel.org>
To: "Matthew Maurer" <mmaurer@...gle.com>, "Danilo Krummrich"
 <dakr@...nel.org>
Cc: "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>, "Greg
 Kroah-Hartman" <gregkh@...uxfoundation.org>, "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 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?

---
Cheers,
Benno